Entropy coding supporting mode switching

ABSTRACT

A decoder for decoding a data stream into which media data is coded has a mode switch configured to activate a low-complexity mode or a high-efficiency mode depending on the data stream, an entropy decoding engine configured to retrieve each symbol of a sequence of symbols by entropy decoding using a selected one of a plurality of entropy decoding schemes, a desymbolizer configured to desymbolize the sequence of symbols to obtain a sequence of syntax elements, a reconstructor configured to reconstruct the media data based on the sequence of syntax elements, selection depending on the activated low-complexity mode or the high-efficiency mode. In another aspect, a desymbolizer is configured to perform desymbolization such that the control parameter varies in accordance with the data stream at a first rate in case of the high-efficiency mode being activated and the control parameter is constant irrespective of the data stream or changes depending on the data stream, but at a second lower rate in case of the low-complexity mode being activated.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 16/693,886 filed Nov. 25, 2019, which is a continuation of U.S.patent application Ser. No. 16/454,247 filed Jun. 27, 2019, now U.S.Pat. No. 10,630,987, which is a continuation of U.S. patent applicationSer. No. 16/259,738, filed Jan. 28, 2019, now U.S. Pat. No. 10,432,939,which is a continuation of U.S. patent application Ser. No. 16/037,914,filed Jul. 17, 2018, now U.S. Pat. No. 10,313,672, which is acontinuation of U.S. patent application Ser. No. 15/843,679, filed Dec.15, 2017, now U.S. Pat. No. 10,057,603, which is a continuation of U.S.patent application Ser. No. 14/108,173 filed Dec. 16, 2013, now U.S.Pat. No. 9,918,090, which is a continuation of International ApplicationPCT/EP2012/061615, filed Jun. 18, 2012 and additionally claims priorityfrom U.S. Provisional Application 61/497,794, filed Jun. 16, 2011, andfrom U.S. Provisional Application 61/508,506, filed Jul. 15, 2011, allof which are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

The present invention is concerned with an entropy coding concept forcoding media content such as video or audio data.

Many audio and video audio codecs are known in the art. Generally, thesecodecs reduce the amount of data necessitated in order to represent themedia content such as audio or video, i.e. they compress the data.However, the demands imposed onto these codecs are not limited toachievement of high compression efficiency. Rather, codecs tend to bespecialized for certain application tasks. Accordingly, in the audiofield, there are audio codecs specialized for speech coding while othersare specialized for coding music. Moreover, in some applications, thecoding delay is critical and, accordingly, some of the codecs arespecialized for low delay. Beyond this, most of these codecs areavailable in different levels of complexity/effectiveness. That is, someof these levels are for lower coding complexity at the cost of lowercoding efficiency. The H.264 video coding standard, for example, offersa baseline profile and a main profile. Primarily, these coding profilesdiffer from each other in activation/deactivation of certain codingoptions/gadgets such as the availability/absence of SBR in the audiocoding field and the availability/absence of B frames in the videocoding field. Beyond this, a considerable part of the complexity ofthese media codecs relates to the entropy coding of the syntax elements.Generally, VLC entropy coding schemes tend to be less complex thanarithmetic coding schemes while the latter show a better codingefficiency. Accordingly, in the H264 standard, context adaptive binaryarithmetic coding (CABAC) is available only in the main profile ratherthan the base line profile. Obviously, base line profile conformdecoders may be configured less complex than main profile conformdecoders. The same applies for the encoders. Since handheld devicesincluding such decoders and/or encoders suffer from a limited energyavailability, the baseline profile has the advantage over the mainprofile with regard to the lower complexity. Main profile conformde/encoders are more complex not only because of the more complexarithmetic coding scheme, but also because of the fact that these mainprofile conform de/encoders have to be backwards compatible withbaseline profile conform data streams. In other words, the increasedcomplexity is due to the arithmetic coding scheme adding up to thecomplexity stemming from the lower complexity variable length codingscheme.

In view of the above, it would be favorable if there would be a codingconcept which allows for a more efficient scalability of the ratio ofthe codex between coding complexity on the one hand and codingefficiency on the other hand.

SUMMARY

According to an embodiment, a decoder for decoding a data stream intowhich media data is coded may have: a mode switch configured to activatea low-complexity mode or a high efficiency mode depending on the datastream; an entropy decoding engine configured to retrieve each symbol ofa sequence of symbols by entropy decoding from the data stream using aselected one of a plurality of entropy decoding schemes; a desymbolizerconfigured to desymbolize the sequence of symbols in order to obtain asequence of syntax elements; a reconstructor configured to reconstructthe media data based on the sequence of syntax elements; wherein theselection depends on the activated one of the low complexity mode andthe high-efficiency mode, wherein the entropy decoding engine isconfigured such that each of the plurality of entropy decoding schemesinvolves arithmetic decoding of the symbols the respective entropydecoding scheme has been selected for, with the plurality of entropydecoding schemes differing from each other in using a differentprobability estimate in the arithmetic decoding and such that theplurality of entropy decoding schemes perform their probabilitysub-division on a common probability interval so as to decode thesymbols from one common bitstream.

According to another embodiment, a decoder for decoding a data streaminto which media data is coded may have: a mode switch configured toactivate a low-complexity mode or a high efficiency mode depending onthe data stream; a desymbolizer configured to desymbolize a sequence ofsymbols obtained from the data stream to obtain integer-valued syntaxelements using a mapping function controllable by a control parameter,for mapping a domain of symbol sequence words to a co-domain of theinteger-valued syntax elements; a reconstructor configured toreconstruct the media data based on the integer-valued syntax elements;wherein the desymbolizer is configured to perform the desymbolizationsuch that the control parameter varies in accordance with the datastream at a first rate in case of the high-efficiency mode beingactivated, and the control parameter is constant irrespective of thedata stream, in case of the low-complexity mode being activated.

According to still another embodiment, a decoder for decoding a datastream into which media data is coded may have: a mode switch configuredto activate a low-complexity mode or a high efficiency mode depending onthe data stream; a desymbolizer configured to desymbolize a sequence ofsymbols obtained from the data stream to obtain integer-valued syntaxelements using a mapping function controllable by a control parameter,for mapping a domain of symbol sequence words to a co-domain of theinteger-valued syntax elements; a reconstructor configured toreconstruct the media data based on the integer-valued syntax elements;wherein the desymbolizer is configured to perform the desymbolizationsuch that the control parameter varies in accordance with the datastream at a first rate in case of the high-efficiency mode beingactivated, and the control parameter changes depending on the datastream at a second rate lower than the first rate, in case of thelow-complexity mode being activated.

According to another embodiment, an encoder for encoding media data intoa data stream may have: an inserter configured to signal within the datastream an activation of a low-complexity mode or a high efficiency mode;a constructor configured to precode the media data into a sequence ofsyntax elements; a symbolizer configured to symbolize the sequence ofsyntax elements into a sequence of symbols; an entropy encoding engineconfigured to encode each symbol of the sequence of symbols into thedatastream using a selected one of a plurality of entropy encodingschemes, wherein the entropy encoding engine is configured to performthe selection depending on the activated one of the low complexity modeand the high-efficiency mode, wherein the entropy encoding engine isconfigured such that each of the plurality of entropy encoding schemesinvolves arithmetic encoding of the symbols the respective entropyencoding scheme has been selected for, with the plurality of entropyencoding schemes differing from each other in using a differentprobability estimate, and such that the plurality of entropy encodingschemes perform their probability sub-division on a common probabilityinterval so as to encode the symbols into a common bitstream.

According to another embodiment, an encoder for encoding media data intoa data stream may have: an inserter configured to signal within the datastream an activation of a low-complexity mode or a high efficiency mode;a constructor configured to precode the media data into a sequence ofsyntax elements having an integer-valued syntax element; a symbolizerconfigured to symbolize the integer-valued syntax element using amapping function controllable by a control parameter, for mapping adomain of integer-valued syntax elements to a co-domain of the symbolsequence words; wherein the symbolizer is configured to perform thesymbolization such that the control parameter varies in accordance withthe data stream at a first rate in case of the high-efficiency modebeing activated and the control parameter is constant irrespective ofthe data stream, in case of the low-complexity mode being activated.

According to still another embodiment, an encoder for encoding mediadata into a data stream may have: an inserter configured to signalwithin the data stream an activation of a low-complexity mode or a highefficiency mode; a constructor configured to precode the media data intoa sequence of syntax elements having an integer-valued syntax element; asymbolizer configured to symbolize the integer-valued syntax elementusing a mapping function controllable by a control parameter, formapping a domain of integer-valued syntax elements to a co-domain of thesymbol sequence words; wherein the symbolizer is configured to performthe symbolization such that the control parameter varies in accordancewith the data stream at a first rate in case of the high-efficiency modebeing activated and the control parameter changes depending on the datastream at a second rate lower than the first rate, in case of thelow-complexity mode being activated.

According to another embodiment, a method for decoding a data streaminto which media data is coded may have the steps of: activating alow-complexity mode or a high efficiency mode depending on the datastream; retrieve each symbol of a sequence of symbols by entropydecoding from the data stream using a selected one of a plurality ofentropy decoding schemes; desymbolizing the sequence of symbols in orderto obtain a sequence of syntax elements; reconstructing the media databased on the sequence of syntax elements; wherein the selection amongthe plurality of entropy decoding schemes is performed depending on theactivated one of the low complexity mode and the high-efficiency mode,wherein the retrieval is performed such that each of the plurality ofentropy decoding schemes involves arithmetic decoding of the symbols therespective entropy decoding scheme has been selected for, with theplurality of entropy decoding schemes differing from each other in usinga different probability estimate in the arithmetic decoding and suchthat the plurality of entropy decoding schemes perform their probabilitysub-division on a common probability interval so as to decode thesymbols from one common bitstream.

According to another embodiment, a method for decoding a data streaminto which media data is coded may have the steps of: activating alow-complexity mode or a high efficiency mode depending on the datastream; desymbolizing a sequence of symbols obtained from the datastream to obtain integer-valued syntax elements using a mapping functioncontrollable by a control parameter, for mapping a domain of symbolsequence words to a co-domain of the integer-valued syntax elements;reconstructing the media data based on the integer-valued syntaxelements, wherein the desymbolization is perform such that the controlparameter varies in accordance with the data stream at a first rate incase of the high-efficiency mode being activated and the controlparameter is constant irrespective of the data stream, in case of thelow-complexity mode being activated.

According to still another embodiment, a method for decoding a datastream into which media data is coded may have the steps of: activatinga low-complexity mode or a high efficiency mode depending on the datastream; desymbolizing a sequence of symbols obtained from the datastream to obtain integer-valued syntax elements using a mapping functioncontrollable by a control parameter, for mapping a domain of symbolsequence words to a co-domain of the integer-valued syntax elements;reconstructing the media data based on the integer-valued syntaxelements, wherein the desymbolization is perform such that the controlparameter varies in accordance with the data stream at a first rate incase of the high-efficiency mode being activated and the controlparameter changes depending on the data stream at a second rate lowerthan the first rate, in case of the low-complexity mode being activated.

According to another embodiment, a method for encoding media data into adata stream may have the steps of: signaling within the data stream anactivation of a low-complexity mode or a high efficiency mode; precodingthe media data into a sequence of syntax elements; symbolizing thesequence of syntax elements into a sequence of symbols; encoding eachsymbol of the sequence of symbols into the datastream using a selectedone of a plurality of entropy encoding schemes, wherein the selectionamong the plurality of entropy encoding schemes is performed dependingon the activated one of the low complexity mode and the high-efficiencymode, wherein the encoding is performed such that each of the pluralityof entropy encoding schemes involves arithmetic encoding of the symbolsthe respective entropy encoding scheme has been selected for, with theplurality of entropy encoding schemes differing from each other in usinga different probability estimate, and such that the plurality of entropyencoding schemes perform their probability sub-division on a commonprobability interval so as to encode the symbols into a commonbitstream.

According to another embodiment, a method for encoding media data into adata stream may have the steps of: signaling within the data stream anactivation of a low-complexity mode or a high efficiency mode; precodingthe media data into a sequence of syntax elements having aninteger-valued syntax element; symbolizing the integer-valued syntaxelement using a mapping function controllable by a control parameter,for mapping a domain of integer-valued syntax elements to a co-domain ofthe symbol sequence words; wherein the symbolization is performed suchthat the control parameter varies in accordance with the data stream ata first rate in case of the high-efficiency mode being activated and thecontrol parameter is constant irrespective of the data stream, in caseof the low-complexity mode being activated.

According to still another embodiment, a method for encoding media datainto a data stream may have the steps of: signaling within the datastream an activation of a low-complexity mode or a high efficiency mode;precoding the media data into a sequence of syntax elements having aninteger-valued syntax element; symbolizing the integer-valued syntaxelement using a mapping function controllable by a control parameter,for mapping a domain of integer-valued syntax elements to a co-domain ofthe symbol sequence words; wherein the symbolization is performed suchthat the control parameter varies in accordance with the data stream ata first rate in case of the high-efficiency mode being activated and thecontrol parameter changes depending on the data stream at a second ratelower than the first rate, in case of the low-complexity mode beingactivated.

Another embodiment may have a computer program having a program code forperforming, when running on a computer, the above methods of decodingand encoding.

In accordance with an embodiment, a decoder for decoding a data streaminto which media data is coded comprises a mode switch configured toactivate a low-complexity mode or a high efficiency mode depending onthe data stream, an entropy decoding engine configured to retrieve eachsymbol of a sequence of symbols by entropy decoding from the data streamusing a selected one of a plurality of entropy decoding schemes, adesymbolizer configured to desymbolize the sequence of symbols in orderto obtain a sequence of syntax elements, a reconstructor configured toreconstruct the media data based on the sequence of syntax elements,wherein the selection depends on the activated one of the low complexitymode and the high-efficiency mode.

In accordance with another embodiment, a decoder for decoding a datastream into which media data is coded comprises a mode switch configuredto activate a low-complexity mode or a high efficiency mode depending onthe data stream, a desymbolizer configured to desymbolize a sequence ofsymbols obtained from the data stream to obtain integer-valued syntaxelements using a mapping function controllable by a control parameter,for mapping a domain of symbol sequence words to a co-domain of theinteger-valued syntax elements, and a reconstructor configured toreconstruct the media data based on the integer-valued syntax elements,wherein the desymbolizer is configured to perform the desymbolizationsuch that the control parameter varies in accordance with the datastream at a first rate in case of the high-efficiency mode beingactivated and the control parameter is constant irrespective of the datastream or changes depending on the data stream, but at a second ratelower than the first rate in case of the low-complexity mode beingactivated.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present application are described in the followingwith respect to the Figures among which

FIG. 1 shows a block diagram of an encoder according to an embodiment;

FIGS. 2 a-2 c schematically show different sub-divisions of a samplearray such as a picture into blocks;

FIG. 3 shows a block diagram of a decoder according to an embodiment;

FIG. 4 shows a block diagram of an encoder according to an embodiment inmore detail;

FIG. 5 shows a block diagram of a decoder according to an embodiment inmore detail;

FIG. 6 schematically illustrates a transform of a block from spatialdomain into spectral domain, the resulting transform block and itsretransformation;

FIG. 7 shows a bock diagram of an encoder according to an embodiment;

FIG. 8 shows a bock diagram of an decoder suitable for decodingbitstream generated by the encoder of FIG. 8 , according to anembodiment;

FIG. 9 shows a schematic diagram illustrating a data packet withmultiplexed partial bitstreams according to an embodiment;

FIG. 10 : shows a schematic diagram illustrating a data packet with analternative segmentation using fixed-size segments according to afurther embodiment;

FIG. 11 shows a bock diagram of an encoder according to an embodimentusing partial bitstream interleaving;

FIG. 12 shows a schematic illustrating examples for the status of acodeword buffer at the encoder side of FIG. 11 according to anembodiment;

FIG. 13 shows a bock diagram of a decoder according to an embodimentusing partial bitstream interleaving;

FIG. 14 shows a bock diagram of a decoder according to an embodimentusing codeword interleaving using a single set of codewords;

FIG. 15 shows a bock diagram of an encoder according to an embodimentusing interleaving of fixed-length bit sequences;

FIG. 16 shows a schematic illustrating examples for the status of aglobal bit buffer at the encoder side of FIG. 15 according to anembodiment;

FIG. 17 shows a bock diagram of a decoder according to an embodimentusing interleaving of fixed-length bit sequences;

FIG. 18 shows a decoder supporting mode switching according to anembodiment;

FIG. 19 shows a decoder supporting mode switching according to a furtherembodiment;

FIG. 20 shows an encoder fitting to decoder of FIG. 18 according to anembodiment;

FIG. 21 shows an encoder fitting to decoder of FIG. 19 according to anembodiment; and

FIG. 22 shows mapping of pStateCtx and fullCtxState/256.

DETAILED DESCRIPTION OF THE INVENTION

It is noted that during the description of the figures, elementsoccurring in several of these Figures are indicated with the samereference sign in each of these Figures and a repeated description ofthese elements as far as the functionality is concerned is avoided inorder to avoid unnecessary repetitions. Nevertheless, thefunctionalities and descriptions provided with respect to one figureshall also apply to other Figures unless the opposite is explicitlyindicated.

In the following, firstly, embodiments of a general video coding conceptare described, with respect to FIG. 1 to 17 . FIG. 1 to 6 relate to thepart of the video codec operating on the syntax level. The followingFIGS. 8 to 17 relate to embodiments for the part of the code relating tothe conversion of the syntax element stream to the data stream and viceversa. Then, specific aspects and embodiments of the present inventionare described in form of possible implementations of the general conceptoutlined with regard to FIG. 1 to 17 . However, it should be noted inadvance, that most of the aspects of the embodiments of the presentinvention are not restricted to video coding. The same applies withregard to many details mentioned below.

FIG. 1 shows an example for an encoder 10 in which aspects of thepresent application may be implemented.

The encoder encodes an array of information samples 20 into a datastream. The array of information samples may represent any kind ofspatially sampled information signal. For example, the sample array 20may be a still picture or a picture of a video. Accordingly, theinformation samples may correspond to brightness values, color values,luma values, chroma values or the like. However, the information samplesmay also be depth values in case of the sample array 20 being a depthmap generated by, for example, a time of light sensor or the like.

The encoder 10 is a block-based encoder. That is, encoder 10 encodes thesample array 20 into the data stream 30 in units of blocks 40. Theencoding in units of blocks 40 does not necessarily mean that encoder 10encodes these blocks 40 totally independent from each other. Rather,encoder 10 may use reconstructions of previously encoded blocks in orderto extrapolate or intra-predict remaining blocks, and may use thegranularity of the blocks for setting coding parameters, i.e. forsetting the way each sample array region corresponding to a respectiveblock is coded.

Further, encoder 10 is a transform coder. That is, encoder 10 encodesblocks 40 by using a transform in order to transfer the informationsamples within each block 40 from spatial domain into spectral domain. Atwo-dimensional transform such as a DCT of FFT or the like may be used.Advantageously, the blocks 40 are of quadratic shape or rectangularshape.

The sub-division of the sample array 20 into blocks 40 shown in FIG. 1merely serves for illustration purposes. FIG. 1 shows the sample array20 as being sub-divided into a regular two-dimensional arrangement ofquadratic or rectangular blocks 40 which abut to each other in anon-overlapping manner. The size of the blocks 40 may be predetermined.That is, encoder 10 may not transfer an information on the block size ofblocks 40 within the data stream 30 to the decoding side. For example,the decoder may expect the predetermined block size.

However, several alternatives are possible. For example, the blocks mayoverlap each other. The overlapping may, however, be restricted to suchan extent that each block has a portion not overlapped by anyneighboring block, or such that each sample of the blocks is overlappedby, at the maximum, one block among the neighboring blocks arranged injuxtaposition to the current block along a predetermined direction. Thelatter would mean that the left and right hand neighbor blocks mayoverlap the current block so as to fully cover the current block butthey may not overlay each other, and the same applies for the neighborsin vertical and diagonal direction.

As a further alternative, the sub-division of sample array 20 intoblocks 40 may be adapted to the content of the sample array 20 by theencoder 10 with the sub-division information on the sub-division usedbeing transferred to the decoder side via bitstream 30.

FIGS. 2 a to 2 c show different examples for a sub-division of a samplearray 20 into blocks 40. FIG. 2 a shows a quadtree-based sub-division ofa sample array 20 into blocks 40 of different sizes, with representativeblocks being indicated at 40 a, 40 b, 40 c and 40 d with increasingsize. In accordance with the sub-division of FIG. 2 a , the sample array20 is firstly divided into a regular two-dimensional arrangement of treeblocks 40 d which, in turn, have individual sub-division informationassociated therewith according to which a certain tree block 40 d may befurther sub-divided according to a quadtree structure or not. The treeblock to the left of block 40 d is exemplarily sub-divided into smallerblocks in accordance with a quadtree structure. The encoder 10 mayperform one two-dimensional transform for each of the blocks shown withsolid and dashed lines in FIG. 2 a . In other words, encoder 10 maytransform the array 20 in units of the block subdivision.

Instead of a quadtree-based sub-division a more general multi tree-basedsub-division may be used and the number of child nodes per hierarchylevel may differ between different hierarchy levels.

FIG. 2 b shows another example for a sub-division. In accordance withFIG. 2 b , the sample array 20 is firstly divided into macroblocks 40 barranged in a regular two-dimensional arrangement in a non-overlappingmutually abutting manner wherein each macroblock 40 b has associatedtherewith sub-division information according to which a macroblock isnot sub-divided, or, if subdivided, sub-divided in a regulartwo-dimensional manner into equally-sized sub-blocks so as to achievedifferent sub-division granularities for different macroblocks. Theresult is a sub-division of the sample array 20 in differently-sizedblocks 40 with representatives of the different sizes being indicated at40 a, 40 b and 40 a′. As in FIG. 2 a , the encoder 10 performs atwo-dimensional transform on each of the blocks shown in FIG. 2 b withthe solid and dashed lines. FIG. 2 c will be discussed later.

FIG. 3 shows a decoder 50 being able to decode the data stream 30generated by encoder 10 to reconstruct a reconstructed version 60 of thesample array 20. Decoder 50 extracts from the data stream 30 thetransform coefficient block for each of the blocks 40 and reconstructsthe reconstructed version 60 by performing an inverse transform on eachof the transform coefficient blocks.

Encoder 10 and decoder 50 may be configured to perform entropyencoding/decoding in order to insert the information on the transformcoefficient blocks into, and extract this information from the datastream, respectively. Details in this regard are described later. Itshould be noted that the data stream 30 not necessarily comprisesinformation on transform coefficient blocks for all the blocks 40 of thesample array 20. Rather, as sub-set of blocks 40 may be coded into thebitstream 30 in another way. For example, encoder 10 may decide torefrain from inserting a transform coefficient block for a certain blockof blocks 40 with inserting into the bitstream 30 alternative codingparameters instead which enable the decoder 50 to predict or otherwisefill the respective block in the reconstructed version 60. For example,encoder 10 may perform a texture analysis in order to locate blockswithin sample array 20 which may be filled at the decoder side bydecoder by way of texture synthesis and indicate this within thebitstream accordingly.

As discussed with respect to the following Figures, the transformcoefficient blocks not necessarily represent a spectral domainrepresentation of the original information samples of a respective block40 of the sample array 20. Rather, such a transform coefficient blockmay represent a spectral domain representation of a prediction residualof the respective block 40. FIG. 4 shows an embodiment for such anencoder. The encoder of FIG. 4 comprises a transform stage 100, anentropy coder 102, an inverse transform stage 104, a predictor 106 and asubtractor 108 as well as an adder 110. Subtractor 108, transform stage100 and entropy coder 102 are serially connected in the order mentionedbetween an input 112 and an output 114 of the encoder of FIG. 4 . Theinverse transform stage 104, adder 110 and predictor 106 are connectedin the order mentioned between the output of transform stage 100 and theinverting input of subtractor 108, with the output of predictor 106 alsobeing connected to a further input of adder 110.

The coder of FIG. 4 is a predictive transform-based block coder. Thatis, the blocks of a sample array 20 entering input 112 are predictedfrom previously encoded and reconstructed portions of the same samplearray 20 or previously coded and reconstructed other sample arrays whichmay precede or succeed the current sample array 20 in presentation time.The prediction is performed by predictor 106. Subtractor 108 subtractsthe prediction from such an original block and the transform stage 100performs a two-dimensional transformation on the prediction residuals.The two-dimensional transformation itself or a subsequent measure insidetransform stage 100 may lead to a quantization of the transformationcoefficients within the transform coefficient blocks. The quantizedtransform coefficient blocks are losslessly coded by, for example,entropy encoding within entropy encoder 102 with the resulting datastream being output at output 114. The inverse transform stage 104reconstructs the quantized residual and adder 110, in turn, combines thereconstructed residual with the corresponding prediction in order toobtain reconstructed information samples based on which predictor 106may predict the afore-mentioned currently encoded prediction blocks.Predictor 106 may use different prediction modes such as intraprediction modes and inter prediction modes in order to predict theblocks and the prediction parameters are forwarded to entropy encoder102 for insertion into the data stream. For each inter-predictedprediction block, respective motion data is inserted into the bitstreamvia entropy encoder 114 in order to enable the decoding side to redo theprediction. The motion data for a prediction block of a picture mayinvolve a syntax portion including a syntax element representing amotion vector difference differentially coding the motion vector for thecurrent prediction block relative to a motion vector predictor derived,for example, by way of a prescribed method from the motion vectors ofneighboring already encoded prediction blocks.

That is, in accordance with the embodiment of FIG. 4 , the transformcoefficient blocks represent a spectral representation of a residual ofthe sample array rather than actual information samples thereof. Thatis, in accordance with the embodiment of FIG. 4 , a sequence of syntaxelements may enter entropy encoder 102 for being entropy encoded intodata stream 114. The sequence of syntax elements may comprise motionvector difference syntax elements for inter-prediction blocks and syntaxelements concerning a significance map indicating positions ofsignificant transform coefficient levels as well as syntax elementsdefining the significant transform coefficient levels themselves, fortransform blocks.

It should be noted that several alternatives exist for the embodiment ofFIG. 4 with some of them having been described within the introductoryportion of the specification which description is incorporated into thedescription of FIG. 4 herewith.

FIG. 5 shows a decoder able to decode a data stream generated by theencoder of FIG. 4 . The decoder of FIG. 5 comprises an entropy decoder150, an inverse transform stage 152, an adder 154 and a predictor 156.Entropy decoder 150, inverse transform stage 152, and adder 154 areserially connected between an input 158 and an output 160 of the decoderof FIG. 5 in the order mentioned. A further output of entropy decoder150 is connected to predictor 156 which, in turn, is connected betweenthe output of adder 154 and a further input thereof. The entropy decoder150 extracts, from the data stream entering the decoder of FIG. 5 atinput 158, the transform coefficient blocks wherein an inverse transformis applied to the transform coefficient blocks at stage 152 in order toobtain the residual signal. The residual signal is combined with aprediction from predictor 156 at adder 154 so as to obtain areconstructed block of the reconstructed version of the sample array atoutput 160. Based on the reconstructed versions, predictor 156 generatesthe predictions thereby rebuilding the predictions performed bypredictor 106 at the encoder side. In order to obtain the samepredictions as those used at the encoder side, predictor 156 uses theprediction parameters which the entropy decoder 150 also obtains fromthe data stream at input 158.

It should be noted that in the above-described embodiments, the spatialgranularity at which the prediction and the transformation of theresidual is performed, do not have to be equal to each other. This isshown in FIG. 2C. This figure shows a sub-division for the predictionblocks of the prediction granularity with solid lines and the residualgranularity with dashed lines. As can be seen, the subdivisions may beselected by the encoder independent from each other. To be more precise,the data stream syntax may allow for a definition of the residualsubdivision independent from the prediction subdivision. Alternatively,the residual subdivision may be an extension of the predictionsubdivision so that each residual block is either equal to or a propersubset of a prediction block. This is shown on FIG. 2 a and FIG. 2 b ,for example, where again the prediction granularity is shown with solidlines and the residual granularity with dashed lines. That is, in FIG. 2a-2 c , all blocks having a reference sign associated therewith would beresidual blocks for which one two-dimensional transform would beperformed while the greater solid line blocks encompassing the dashedline blocks 40 a, for example, would be prediction blocks for which aprediction parameter setting is performed individually.

The above embodiments have in common that a block of (residual ororiginal) samples is to be transformed at the encoder side into atransform coefficient block which, in turn, is to be inverse transformedinto a reconstructed block of samples at the decoder side. This isillustrated in FIG. 6 . FIG. 6 shows a block of samples 200. In case ofFIG. 6 , this block 200 is exemplarily quadratic and 4×4 samples 202 insize. The samples 202 are regularly arranged along a horizontaldirection x and vertical direction y. By the above-mentionedtwo-dimensional transform T, block 200 is transformed into spectraldomain, namely into a block 204 of transform coefficients 206, thetransform block 204 being of the same size as block 200. That is,transform block 204 has as many transform coefficients 206 as block 200has samples, in both horizontal direction and vertical direction.However, as transform T is a spectral transformation, the positions ofthe transform coefficients 206 within transform block 204 do notcorrespond to spatial positions but rather to spectral components of thecontent of block 200. In particular, the horizontal axis of transformblock 204 corresponds to an axis along which the spectral frequency inthe horizontal direction monotonically increases while the vertical axiscorresponds to an axis along which the spatial frequency in the verticaldirection monotonically increases wherein the DC component transformcoefficient is positioned in a corner—here exemplarily the top leftcorner—of block 204 so that at the bottom right-hand corner, thetransform coefficient 206 corresponding to the highest frequency in bothhorizontal and vertical direction is positioned. Neglecting the spatialdirection, the spatial frequency to which a certain transformcoefficient 206 belongs, generally increases from the top left corner tothe bottom right-hand corner. By an inverse transform T⁻¹, the transformblock 204 is re-transferred from spectral domain to spatial domain, soas to re-obtain a copy 208 of block 200. In case no quantization/losshas been introduced during the transformation, the reconstruction wouldbe perfect.

As already noted above, it may be seen from FIG. 6 that greater blocksizes of block 200 increase the spectral resolution of the resultingspectral representation 204. On the other hand, quantization noise tendsto spread over the whole block 208 and thus, abrupt and very localizedobjects within blocks 200 tend to lead to deviations of there-transformed block relative to the original block 200 due toquantization noise. The main advantage of using greater blocks is,however, that the ratio between the number of significant, i.e. non-zero(quantized) transform coefficients, i.e. levels, on the one hand and thenumber of insignificant transform coefficients on the other hand may bedecreased within larger blocks compared to smaller blocks therebyenabling a better coding efficiency. In other words, frequently, thesignificant transform coefficient levels, i.e. the transformcoefficients not quantized to zero, are distributed over the transformblock 204 sparsely. Due to this, in accordance with the embodimentsdescribed in more detail below, the positions of the significanttransform coefficient levels is signaled within the data stream by wayof a significance map. Separately therefrom, the values of thesignificant transform coefficient, i.e., the transform coefficientlevels in case of the transform coefficients being quantized, aretransmitted within the data stream.

All the encoders and decoders described above, are, thus, configured todeal with a certain syntax of syntax elements. That is, theafore-mentioned syntax elements such as the transform coefficientlevels, syntax elements concerning the significance map of transformblocks, the motion data syntax elements concerning inter-predictionblocks and so on are assumed to be sequentially arranged within the datastream in a prescribed way. Such a prescribed way may be represented inform of a pseudo code as it is done, for example, in the H.264 standardor other audio/video codecs.

In even other words, the above description, primarily dealt with theconversion of media data, here exemplarily video data, to a sequence ofsyntax elements in accordance with a predefined syntax structureprescribing certain syntax element types, its semantics and the orderamong them. The entropy encoder and entropy decoder of FIGS. 4 and 5 ,may be configured to operate, and may be structured, as outlined next.Same are responsible for performing the conversion between syntaxelement sequence and data stream, i.e. symbol or bit stream.

An entropy encoder according to an embodiment is illustrated in FIG. 7 .The encoder losslessly converts a stream of syntax elements 301 into aset of two or more partial bitstreams 312.

In an embodiment of the invention, each syntax element 301 is associatedwith a category of a set of one or more categories, i.e. a syntaxelement type. As an example, the categories can specify the type of thesyntax element. In the context of hybrid video coding, a separatecategory may be associated with macroblock coding modes, block codingmodes, reference picture indices, motion vector differences, subdivisionflags, coded block flags, quantization parameters, transform coefficientlevels, etc. In other application areas such as audio, speech, text,document, or general data coding, different categorizations of syntaxelements are possible.

In general, each syntax element can take a value of a finite orcountable infinite set of values, where the set of possible syntaxelement values can differ for different syntax element categories. Forexample, there are binary syntax elements as well as integer-valuedones.

For reducing the complexity of the encoding and decoding algorithm andfor allowing a general encoding and decoding design for different syntaxelements and syntax element categories, the syntax elements 301 areconverted into ordered sets of binary decisions and these binarydecisions are then processed by simple binary coding algorithms.Therefore, the binarizer 302 bijectively maps the value of each syntaxelement 301 onto a sequence (or string or word) of bins 303. Thesequence of bins 303 represents a set of ordered binary decisions. Eachbin 303 or binary decision can take one value of a set of two values,e.g. one of the values 0 and 1. The binarization scheme can be differentfor different syntax element categories. The binarization scheme for aparticular syntax element category can depend on the set of possiblesyntax element values and/or other properties of the syntax element forthe particular category.

Table 1 illustrates three example binarization schemes for countableinfinite sets. Binarization schemes for countable infinite sets can alsobe applied for finite sets of syntax element values. In particular forlarge finite sets of syntax element values, the inefficiency (resultingfrom unused sequences of bins) can be negligible, but the universalityof such binarization schemes provides an advantage in terms ofcomplexity and memory requirements. For small finite sets of syntaxelement values, it is often of advantage (in terms of coding efficiency)to adapt the binarization scheme to the number of possible symbolvalues.

Table 2 illustrates three example binarization schemes for finite setsof 8 values. Binarization schemes for finite sets can be derived fromthe universal binarization schemes for countable infinite sets bymodifying some sequences of bins in a way that the finite sets of binsequences represent a redundancy-free code (and potentially reorderingthe bin sequences). As an example, the truncated unary binarizationscheme in Table 2 was created by modifying the bin sequence for thesyntax element 7 of the universal unary binarization (see Table 1). Thetruncated and reordered Exp-Golomb binarization of order 0 in Table 2was created by modifying the bin sequence for the syntax element 7 ofthe universal Exp-Golomb order 0 binarization (see Table 1) and byreordering the bin sequences (the truncated bin sequence for symbol 7was assigned to symbol 1). For finite sets of syntax elements, it isalso possible to use non-systematic/non-universal binarization schemes,as exemplified in the last column of Table 2.

TABLE 1 Binarization examples for countable infinite sets (or largefinite sets). Exp-Golomb Exp-Golomb symbol unary order 0 order 1 valuebinarization binarization binarization 0 1 1 10 1 01 010 11 2 001 0110100 3 0001 0010 0 0101 4 0000 1 0010 1 0110 5 0000 01 0011 0 0111 60000 001 0011 1 0010 00 7 0000 0001 0001 000 0010 01 . . . . . . . . . .. .

TABLE 2 Binarization examples for finite sets. truncated and reorderedtruncated Exp-Golomb symbol unary order 0 non-systematic valuebinarization binarization binarization 0 1 1 000 1 01 000 001 2 001 01001 3 0001 011 1000 4 0000 1 0010 0 1001 5 0000 01 0010 1 1010 6 0000 0010011 0 1011 0 7 0000 000 0011 1 1011 1

Each bin 303 of the sequence of bins created by the binarizer 302 is fedinto the parameter assigner 304 in sequential order. The parameterassigner assigns a set of one or more parameters to each bin 303 andoutputs the bin with the associated set of parameters 305. The set ofparameters is determined in exactly the same way at encoder and decoder.The set of parameters may consist of one or more of the followingparameters:

In particular, parameter assigner 304 may be configured to assign to acurrent bin 303 a context model. For example, parameter assigner 304 mayselect one of available context indices for the current bin 303. Theavailable set of contexts for a current bin 303 may depend on the typeof the bin which, in turn, may be defined by the type/category of thesyntax element 301, the binarization of which the current bin 303 ispart of, and a position of the current bin 303 within the latterbinarization. The context selection among the available context set maydepend on previous bins and the syntax elements associated with thelatter. Each of these contexts has a probability model associatedtherewith, i.e. a measure for an estimate of the probability for one ofthe two possible bin values for the current bin. The probability modelmay in particular be a measure for an estimate of the probability forthe less probable or more probable bin value for the current bin, with aprobability model additionally being defined by an identifier specifyingan estimate for which of the two possible bin values represents the lessprobable or more probable bin value for the current bin 303. In case ofmerely one context being available for the current bin, the contextselection may be left away. As will be outlined in more detail below,parameter assigner 304 may also perform a probability model adaptationin order to adapt the probability models associated with the variouscontexts to the actual bin statistics of the respective bins belongingto the respective contexts.

As will also be described in more detail below, parameter assigner 304may operate differently depending on a high efficiency (HE) mode or lowcomplexity (LC) mode being activated. In both modes the probabilitymodel associates the current bin 303 to any of the bin encoders 310 aswill be outlined below, but the mode of operation of the parameterassigner 304 tends to be less complex in the LC mode with, however, thecoding efficiency being increased in the high efficiency mode due to theparameter assigner 304 causing the association of the individual bins303 to the individual encoders 310 to be more accurately adapted to thebin statistics, thereby optimizing the entropy relative to the LC mode.

Each bin with an associated set of parameters 305 that is output of theparameter assigner 304 is fed into a bin buffer selector 306. The binbuffer selector 306 potentially modifies the value of the input bin 305based on the input bin value and the associated parameters 305 and feedsthe output bin 307—with a potentially modified value—into one of two ormore bin buffers 308. The bin buffer 308 to which the output bin 307 issent is determined based on the value of the input bin 305 and/or thevalue of the associated parameters 305.

In an embodiment of the invention, the bin buffer selector 306 does notmodify the value of the bin, i.e., the output bin 307 has the same valueas the input bin 305. In a further embodiment of the invention, the binbuffer selector 306 determines the output bin value 307 based on theinput bin value 305 and the associated measure for an estimate of theprobability for one of the two possible bin values for the current bin.In an embodiment of the invention, the output bin value 307 is set equalto the input bin value 305 if the measure for the probability for one ofthe two possible bin values for the current bin is less than (or lessthan or equal to) a particular threshold; if the measure for theprobability for one of the two possible bin values for the current binis greater than or equal to (or greater than) a particular threshold,the output bin value 307 is modified (i.e., it is set to the opposite ofthe input bin value). In a further embodiment of the invention, theoutput bin value 307 is set equal to the input bin value 305 if themeasure for the probability for one of the two possible bin values forthe current bin is greater than (or greater than or equal to) aparticular threshold; if the measure for the probability for one of thetwo possible bin values for the current bin is less than or equal to (orless than) a particular threshold, the output bin value 307 is modified(i.e., it is set to the opposite of the input bin value). In anembodiment of the invention, the value of the threshold corresponds to avalue of 0.5 for the estimated probability for both possible bin values.

In a further embodiment of the invention, the bin buffer selector 306determines the output bin value 307 based on the input bin value 305 andthe associated identifier specifying an estimate for which of the twopossible bin values represents the less probable or more probable binvalue for the current bin. In an embodiment of the invention, the outputbin value 307 is set equal to the input bin value 305 if the identifierspecifies that the first of the two possible bin values represents theless probable (or more probable) bin value for the current bin, and theoutput bin value 307 is modified (i.e., it is set to the opposite of theinput bin value) if identifier specifies that the second of the twopossible bin values represents the less probable (or more probable) binvalue for the current bin.

In an embodiment of the invention, the bin buffer selector 306determines the bin buffer 308 to which the output bin 307 is sent basedon the associated measure for an estimate of the probability for one ofthe two possible bin values for the current bin. In an embodiment of theinvention, the set of possible values for the measure for an estimate ofthe probability for one of the two possible bin values is finite and thebin buffer selector 306 contains a table that associates exactly one binbuffer 308 with each possible value for the estimate of the probabilityfor one of the two possible bin values, where different values for themeasure for an estimate of the probability for one of the two possiblebin values can be associated with the same bin buffer 308. In a furtherembodiment of the invention, the range of possible values for themeasure for an estimate of the probability for one of the two possiblebin values is partitioned into a number of intervals, the bin bufferselector 306 determines the interval index for the current measure foran estimate of the probability for one of the two possible bin values,and the bin buffer selector 306 contains a table that associates exactlyone bin buffer 308 with each possible value for the interval index,where different values for the interval index can be associated with thesame bin buffer 308. In an embodiment of the invention, input bins 305with opposite measures for an estimate of the probability for one of thetwo possible bin values (opposite measure are those which representprobability estimates P and 1−P) are fed into the same bin buffer 308.In a further embodiment of the invention, the association of the measurefor an estimate of the probability for one of the two possible binvalues for the current bin with a particular bin buffer is adapted overtime, e.g. in order to ensure that the created partial bitstreams havesimilar bit rates. Further below, the interval index will also be calledpipe index, while the pipe index along with a refinement index and aflag indicating the more probable bin value indexes the actualprobability model, i.e. the probability estimate.

In a further embodiment of the invention, the bin buffer selector 306determines the bin buffer 308 to which the output bin 307 is sent basedon the associated measure for an estimate of the probability for theless probable or more probable bin value for the current bin. In anembodiment of the invention, the set of possible values for the measurefor an estimate of the probability for the less probable or moreprobable bin value is finite and the bin buffer selector 306 contains atable that associates exactly one bin buffer 308 with each possiblevalue of the estimate of the probability for the less probable or moreprobable bin value, where different values for the measure for anestimate of the probability for the less probable or more probable binvalue can be associated with the same bin buffer 308. In a furtherembodiment of the invention, the range of possible values for themeasure for an estimate of the probability for the less probable or moreprobable bin value is partitioned into a number of intervals, the binbuffer selector 306 determines the interval index for the currentmeasure for an estimate of the probability for the less probable or moreprobable bin value, and the bin buffer selector 306 contains a tablethat associates exactly one bin buffer 308 with each possible value forthe interval index, where different values for the interval index can beassociated with the same bin buffer 308. In a further embodiment of theinvention, the association of the measure for an estimate of theprobability for the less probable or more probable bin value for thecurrent bin with a particular bin buffer is adapted over time, e.g. inorder to ensure that the created partial bitstreams have similar bitrates.

Each of the two or more bin buffers 308 is connected with exactly onebin encoder 310 and each bin encoder is only connected with one binbuffer 308. Each bin encoder 310 reads bins from the associated binbuffer 308 and converts a sequence of bins 309 into a codeword 311,which represents a sequence of bits. The bin buffers 308 representfirst-in-first-out buffers; bins that are fed later (in sequentialorder) into a bin buffer 308 are not encoded before bins that are fedearlier (in sequential order) into the bin buffer. The codewords 311that are output of a particular bin encoder 310 are written to aparticular partial bitstream 312. The overall encoding algorithmconverts syntax elements 301 into two or more partial bitstreams 312,where the number of partial bitstreams is equal to the number of binbuffers and bin encoders. In an embodiment of the invention, a binencoder 310 converts a variable number of bins 309 into a codeword 311of a variable number of bits. One advantage of the above- andbelow-outlined embodiments of the invention is that the encoding of binscan be done in parallel (e.g. for different groups of probabilitymeasures), which reduces the processing time for severalimplementations.

Another advantage of embodiments of the invention is that the binencoding, which is done by the bin encoders 310, can be specificallydesigned for different sets of parameters 305. In particular, the binencoding and encoding can be optimized (in terms of coding efficiencyand/or complexity) for different groups of estimated probabilities. Onthe one hand side, this allows a reduction of the encoding/decodingcomplexity, and on the other hand side, it allows an improvement of thecoding efficiency. In an embodiment of the invention, the bin encoders310 implement different encoding algorithms (i.e. mapping of binsequences onto codewords) for different groups of measures for anestimate of the probability for one of the two possible bin values 305for the current bin. In a further embodiment of the invention, the binencoders 310 implement different encoding algorithms for differentgroups of measures for an estimate of the probability for the lessprobable or more probable bin value for the current bin.

In an embodiment of the invention, the bin encoders 310—or one or moreof the bin encoders—represent entropy encoders that directly mapsequences of input bins 309 onto codewords 310. Such mappings can beefficiently implemented and do not require a complex arithmetic codingengine. The inverse mapping of codewords onto sequences of bins (as donein the decoder) should to be unique in order to guarantee perfectdecoding of the input sequence, but the mapping of bin sequences 309onto codewords 310 doesn't necessarily need to be unique, i.e., it ispossible that a particular sequence of bins can be mapped onto more thanone sequence of codewords. In an embodiment of the invention, themapping of sequences of input bins 309 onto codewords 310 is bijective.In a further embodiment of the invention, the bin encoders 310—or one ormore of the bin encoders—represent entropy encoders that directly mapvariable-length sequences of input bins 309 onto variable-lengthcodewords 310. In an embodiment of the invention, the output codewordsrepresent redundancy-free codes such as general huffman codes orcanonical huffman codes.

Two examples for the bijective mapping of bin sequences toredundancy-free codes are illustrated in Table 3. In a furtherembodiment of the invention, the output codewords represent redundantcodes suitable for error detection and error recovery. In a furtherembodiment of the invention, the output codewords represent encryptioncodes suitable for encrypting the syntax elements.

TABLE 3 Examples for mappings between bin sequences and codewords.sequence of bins codewords (bin order is from left to right) (bits orderis from left to right) 0000 0000 1 0000 0001 0000 0000 001 0001 0000 010010 0000 1 0011 0001 0100 001 0101 01 0110 1 0111 sequence of binscodewords (bin order is from left to right) (bits order is from left toright) 000 10 01 11 001 010 11 011 1000 0 0001 1001 0010 1010 0011 10001 0000 0 1011 0000 1

In a further embodiment of the invention, the bin encoders 310—or one ormore of the bin encoders—represent entropy encoders that directly mapvariable-length sequences of input bins 309 onto fixed-length codewords310. In a further embodiment of the invention, the bin encoders 310—orone or more of the bin encoders—represent entropy encoders that directlymap fixed-length sequences of input bins 309 onto variable-lengthcodewords 310.

The decoder according an embodiment of the invention is illustrated inFIG. 8 . The decoder performs basically the inverse operations of theencoder, so that the (previously encoded) sequence of syntax elements327 is decoded from a set of two or more partial bitstreams 324. Thedecoder includes two different process flows: A flow for data requests,which replicates the data flow of the encoder, and a data flow, whichrepresents the inverse of the encoder data flow. In the illustration inFIG. 8 , the dashed arrows represent the data request flow, while thesolid arrows represent the data flow. The building blocks of the decoderbasically replicate the building blocks of the encoder, but implementthe inverse operations.

The decoding of a syntax element is triggered by a request for a newdecoded syntax element 313 that is sent to the binarizer 314. In anembodiment of the invention, each request for a new decoded syntaxelement 313 is associated with a category of a set of one or morecategories. The category that is associated with a request for a syntaxelement is the same as the category that was associated with thecorresponding syntax element during encoding.

The binarizer 314 maps the request for a syntax element 313 into one ormore requests for a bin that are sent to the parameter assigner 316. Asfinal response to a request for a bin that is sent to the parameterassigner 316 by the binarizer 314, the binarizer 314 receives a decodedbin 326 from the bin buffer selector 318. The binarizer 314 compares thereceived sequence of decoded bins 326 with the bin sequences of aparticular binarization scheme for the requested syntax element and, ifthe received sequence of decoded bins 26 matches the binarization of asyntax element, the binarizer empties its bin buffer and outputs thedecoded syntax element as final response to the request for a newdecoded symbol. If the already received sequence of decoded bins doesnot match any of the bin sequences for the binarization scheme for therequested syntax element, the binarizer sends another request for a binto the parameter assigner until the sequence of decoded bins matches oneof the bin sequences of the binarization scheme for the requested syntaxelement. For each request for a syntax element, the decoder uses thesame binarization scheme that was used for encoding the correspondingsyntax element. The binarization scheme can be different for differentsyntax element categories. The binarization scheme for a particularsyntax element category can depend on the set of possible syntax elementvalues and/or other properties of the syntax elements for the particularcategory.

The parameter assigner 316 assigns a set of one or more parameters toeach request for a bin and sends the request for a bin with theassociated set of parameters to the bin buffer selector. The set ofparameters that are assigned to a requested bin by the parameterassigner is the same that was assigned to the corresponding bin duringencoding. The set of parameters may consist of one or more of theparameters that are mentioned in the encoder description of FIG. 7 .

In an embodiment of the invention, the parameter assigner 316 associateseach request for a bin with the same parameters as assigner 304 did,i.e. a context and its associated measure for an estimate of theprobability for one of the two possible bin values for the currentrequested bin, such as a measure for an estimate of the probability forthe less probable or more probable bin value for the current requestedbin and an identifier specifying an estimate for which of the twopossible bin values represents the less probable or more probable binvalue for the current requested bin.

The parameter assigner 316 may determine one or more of the abovementioned probability measures (measure for an estimate of theprobability for one of the two possible bin values for the currentrequested bin, measure for an estimate of the probability for the lessprobable or more probable bin value for the current requested bin,identifier specifying an estimate for which of the two possible binvalues represents the less probable or more probable bin value for thecurrent requested bin) based on a set of one or more already decodedsymbols. The determination of the probability measures for a particularrequest for a bin replicates the process at the encoder for thecorresponding bin. The decoded symbols that are used for determining theprobability measures can include one or more already decoded symbols ofthe same symbol category, one or more already decoded symbols of thesame symbol category that correspond to data sets (such as blocks orgroups of samples) of neighboring spatial and/or temporal locations (inrelation to the data set associated with the current request for asyntax element), or one or more already decoded symbols of differentsymbol categories that correspond to data sets of the same and/orneighboring spatial and/or temporal locations (in relation to the dataset associated with the current request for a syntax element).

Each request for a bin with an associated set of parameters 317 that isoutput of the parameter assigner 316 is fed into a bin buffer selector318. Based on the associated set of parameters 317, the bin bufferselector 318 sends a request for a bin 319 to one of two or more binbuffers 320 and receives a decoded bin 325 from the selected bin buffer320. The decoded input bin 325 is potentially modified and the decodedoutput bin 326—with a potentially modified value—is send to thebinarizer 314 as final response to the request for a bin with anassociated set of parameters 317.

The bin buffer 320 to which the request for a bin is forwarded isselected in the same way as the bin buffer to which the output bin ofthe bin buffer selector at the encoder side was sent.

In an embodiment of the invention, the bin buffer selector 318determines the bin buffer 320 to which the request for a bin 319 is sentbased on the associated measure for an estimate of the probability forone of the two possible bin values for the current requested bin. In anembodiment of the invention, the set of possible values for the measurefor an estimate of the probability for one of the two possible binvalues is finite and the bin buffer selector 318 contains a table thatassociates exactly one bin buffer 320 with each possible value of theestimate of the probability for one of the two possible bin values,where different values for the measure for an estimate of theprobability for one of the two possible bin values can be associatedwith the same bin buffer 320. In a further embodiment of the invention,the range of possible values for the measure for an estimate of theprobability for one of the two possible bin values is partitioned into anumber of intervals, the bin buffer selector 318 determines the intervalindex for the current measure for an estimate of the probability for oneof the two possible bin values, and the bin buffer selector 318 containsa table that associates exactly one bin buffer 320 with each possiblevalue for the interval index, where different values for the intervalindex can be associated with the same bin buffer 320. In an embodimentof the invention, requests for bins 317 with opposite measures for anestimate of the probability for one of the two possible bin values(opposite measure are those which represent probability estimates P and1−P) are forwarded to the same bin buffer 320. In a further embodimentof the invention, the association of the measure for an estimate of theprobability for one of the two possible bin values for the current binrequest with a particular bin buffer is adapted over time.

In a further embodiment of the invention, the bin buffer selector 318determines the bin buffer 320 to which the request for a bin 319 is sentbased on the associated measure for an estimate of the probability forthe less probable or more probable bin value for the current requestedbin. In an embodiment of the invention, the set of possible values forthe measure for an estimate of the probability for the less probable ormore probable bin value is finite and the bin buffer selector 318contains a table that associates exactly one bin buffer 320 with eachpossible value of the estimate of the probability for the less probableor more probable bin value, where different values for the measure foran estimate of the probability for the less probable or more probablebin value can be associated with the same bin buffer 320. In a furtherembodiment of the invention, the range of possible values for themeasure for an estimate of the probability for the less probable or moreprobable bin value is partitioned into a number of intervals, the binbuffer selector 318 determines the interval index for the currentmeasure for an estimate of the probability for the less probable or moreprobable bin value, and the bin buffer selector 318 contains a tablethat associates exactly one bin buffer 320 with each possible value forthe interval index, where different values for the interval index can beassociated with the same bin buffer 320. In a further embodiment of theinvention, the association of the measure for an estimate of theprobability for the less probable or more probable bin value for thecurrent bin request with a particular bin buffer is adapted over time.

After receiving a decoded bin 325 from the selected bin buffer 320, thebin buffer selector 318 potentially modifies the input bin 325 and sendsthe output bin 326—with a potentially modified value—to the binarizer314. The input/output bin mapping of the bin buffer selector 318 is theinverse of the input/output bin mapping of the bin buffer selector atthe encoder side.

In an embodiment of the invention, the bin buffer selector 318 does notmodify the value of the bin, i.e., the output bin 326 has the same valueas the input bin 325. In a further embodiment of the invention, the binbuffer selector 318 determines the output bin value 326 based on theinput bin value 325 and the measure for an estimate of the probabilityfor one of the two possible bin values for the current requested binthat is associated with the request for a bin 317. In an embodiment ofthe invention, the output bin value 326 is set equal to the input binvalue 325 if the measure for the probability for one of the two possiblebin values for the current bin request is less than (or less than orequal to) a particular threshold; if the measure for the probability forone of the two possible bin values for the current bin request isgreater than or equal to (or greater than) a particular threshold, theoutput bin value 326 is modified (i.e., it is set to the opposite of theinput bin value). In a further embodiment of the invention, the outputbin value 326 is set equal to the input bin value 325 if the measure forthe probability for one of the two possible bin values for the currentbin request is greater than (or greater than or equal to) a particularthreshold; if the measure for the probability for one of the twopossible bin values for the current bin request is less than or equal to(or less than) a particular threshold, the output bin value 326 ismodified (i.e., it is set to the opposite of the input bin value). In anembodiment of the invention, the value of the threshold corresponds to avalue of 0.5 for the estimated probability for both possible bin values.

In a further embodiment of the invention, the bin buffer selector 318determines the output bin value 326 based on the input bin value 325 andthe identifier, specifying an estimate for which of the two possible binvalues represents the less probable or more probable bin value for thecurrent bin request, that is associated with the request for a bin 317.In an embodiment of the invention, the output bin value 326 is set equalto the input bin value 325 if the identifier specifies that the first ofthe two possible bin values represents the less probable (or moreprobable) bin value for the current bin request, and the output binvalue 326 is modified (i.e., it is set to the opposite of the input binvalue) if identifier specifies that the second of the two possible binvalues represents the less probable (or more probable) bin value for thecurrent bin request.

As described above, the bin buffer selector sends a request for a bin319 to one of the two or more bin buffers 320. The bin buffers 20represent first-in-first-out buffers, which are fed with sequences ofdecoded bins 321 from the connected bin decoders 322. As response to arequest for a bin 319 that is sent to a bin buffer 320 from the binbuffer selector 318, the bin buffer 320 removes the bin of its contentthat was first fed into the bin buffer 320 and sends it to the binbuffer selector 318. Bins that are earlier sent to the bin buffer 320are earlier removed and sent to the bin buffer selector 318.

Each of the two or more bin buffers 320 is connected with exactly onebin decoder 322 and each bin decoder is only connected with one binbuffer 320. Each bin decoder 322 reads codewords 323, which representsequences of bits, from a separate partial bitstream 324. The bindecoder converts a codeword 323 into a sequence of bins 321 that is sentto the connected bin buffer 320. The overall decoding algorithm convertstwo or more partial bitstreams 324 into a number of decoded syntaxelements, where the number of partial bitstreams is equal to the numberof bin buffers and bin decoders and the decoding of syntax elements istriggered by requests for new syntax elements. In an embodiment of theinvention, a bin decoder 322 converts codewords 323 of a variable numberof bits into a sequence of a variable number of bins 321. One advantageof embodiments of the invention is that the decoding of bins from thetwo or more partial bitstreams can be done in parallel (e.g. fordifferent groups of probability measures), which reduces the processingtime for several implementations.

Another advantage of embodiments of the invention is that the bindecoding, which is done by the bin decoders 322, can be specificallydesigned for different sets of parameters 317. In particular, the binencoding and decoding can be optimized (in terms of coding efficiencyand/or complexity) for different groups of estimated probabilities. Onthe one hand side, this allows a reduction of the encoding/decodingcomplexity relative to state-of-the-art entropy coding algorithms withsimilar coding efficiency. On the other hand side, it allows animprovement of the coding efficiency relative to state-of-the-artentropy coding algorithms with similar encoding/decoding complexity. Inan embodiment of the invention, the bin decoders 322 implement differentdecoding algorithms (i.e. mapping of bin sequences onto codewords) fordifferent groups of measures for an estimate of the probability for oneof the two possible bin values 317 for the current bin request. In afurther embodiment of the invention, the bin decoders 322 implementdifferent decoding algorithms for different groups of measures for anestimate of the probability for the less probable or more probable binvalue for the current requested bin.

The bin decoders 322 do the inverse mapping of the corresponding binencoders at the encoder side.

In an embodiment of the invention, the bin decoders 322—or one or moreof the bin decoders—represent entropy decoders that directly mapcodewords 323 onto sequences of bins 321. Such mappings can beefficiently implemented and do not require a complex arithmetic codingengine. The mapping of codewords onto sequences of bins has to beunique. In an embodiment of the invention, the mapping of codewords 323onto sequences of bins 321 is bijective. In a further embodiment of theinvention, the bin decoders 310—or one or more of the bindecoders—represent entropy decoders that directly map variable-lengthcodewords 323 into variable-length sequences of bins 321. In anembodiment of the invention, the input codewords representredundancy-free codes such as general huffman codes or canonical huffmancodes. Two examples for the bijective mapping of redundancy-free codesto bin sequences are illustrated in Table 3.

In a further embodiment of the invention, the bin decoders 322—or one ormore of the bin decoders—represent entropy decoders that directly mapfixed-length codewords 323 onto variable-length sequences of bins 321.In a further embodiment of the invention, the bin decoders 322—or one ormore of the bin decoders—represent entropy decoders that directly mapvariable-length codewords 323 onto fixed-length sequences of bins 321.

Thus, FIGS. 7 and 8 showed an embodiment for an encoder for encoding asequence of symbols 3 and a decoder for reconstructing same. The encodercomprises an assigner 304 configured to assign a number of parameters305 to each symbol of the sequence of symbols. The assignment is basedon information contained within previous symbols of the sequence ofsymbols such as the category of the syntax element 1 to therepresentation—such as binarization—of which the current symbol belongsand which, according to the syntax structure of the syntax elements 1,is currently be expected which expectation, in turn, is deducible fromthe history of previous syntax elements 1 and symbols 3. Further, theencoder comprises a plurality of entropy encoders 10 each of which isconfigured to convert the symbols 3 forwarded to the respective entropyencoder into a respective bitstream 312, and a selector 306 configuredto forward each symbol 3 to a selected one of the plurality of entropyencoders 10, the selection depending on the number of parameters 305assigned to the respective symbol 3. The assignor 304 could be thoughtof as being integrated into selector 206 in order to yield a respectiveselector 502.

The decoder for reconstructing a sequence of symbols comprises aplurality of entropy decoders 322, each of which is configured toconvert a respective bitstream 323 into symbols 321; an assigner 316configured to assign a number of parameters 317 to each symbol 315 of asequence of symbols to be reconstructed based on information containedwithin previously reconstructed symbols of the sequence of symbols (see326 and 327 in FIG. 8 ); and a selector 318 configured to retrieve eachsymbol of the sequence of symbols to be reconstructed from a selectedone of the plurality of entropy decoders 322, the selection depending onthe number of parameters defined to the respective symbol. The assigner316 may be configured such that the number of parameters assigned toeach symbol comprises, or is, a measure for an estimate of a probabilityof distribution among the possible symbol values a respective symbol mayassume. Again, assignor 316 and selector 318 may be thought of asintegrated into one block, a selector 402. The sequence of symbols to bereconstructed may be of a binary alphabet and the assigner 316 may beconfigured such that the estimate of the probability distributionconsists of a measure for an estimate of a probability of a lessprobable or more probable bin value of the two possible bin values ofthe binary alphabet and an identifier specifying an estimate for whichof the two possible bin values represents the less probable or moreprobable bin value. The assigner 316 may further be configured tointernally assign a context to each symbol of the sequence of symbols315 to be reconstructed based on the information contained withinpreviously reconstructed symbols of the sequence of symbols to bereconstructed with each context having a respective probabilitydistribution estimate associated therewith, and to adapt the probabilitydistribution estimate for each context to an actual symbol statisticbased on symbol values of previously reconstructed symbols to which therespective context is assigned. The context may take into account aspatial relationship or neighborhood of positions to which the syntaxelements belong such as in video or picture coding, or even in tables incase of financial applications. Then, the measure for the estimate ofthe probability distribution for each symbol may be determined based onthe probability distribution estimate associated with the contextassigned to the respective symbol such as by quantizing, or using as anindex into a respective table, the probability distribution estimateassociated with the context assigned with the respective symbol (in thebelow embodiments indexed by a pipe index along with a refinement index)to one of a plurality of probability distribution estimaterepresentatives (clipping away the refinement index) in order to obtainthe measure for the estimate of the probability distribution (the pipeindex indexing the partial bitstream 312). The selector may beconfigured such that a bijective association is defined between theplurality of entropy encoders and the plurality of probabilitydistribution estimate representatives. The selector 18 may be configuredto change a quantization mapping from a range of the probabilitydistribution estimates to the plurality of probability distributionestimate representatives in a predetermined deterministic way dependingon previously reconstructed symbols of the sequence of symbols, overtime. That is, selector 318 may change the quantization step sizes, i.e.the intervals of probability distributions mapped onto the individualprobability indices bijectively associated with the individual entropydecoders. The plurality of entropy decoders 322, in turn, may beconfigured to adapt their way of converting symbols into bit streamsresponsive to a change in the quantization mapping. For example, eachentropy decoder 322 may be optimized for, i.e. may have an optimalcompression rate for, a certain probability distribution estimate withinthe respective probability distribution estimate quantization interval,and may change its codeword/symbol sequence mapping so as to adapt theposition of this certain probability distribution estimate within therespective probability distribution estimate quantization interval upona change of the latter so as to be optimized. The selector may beconfigured to change the quantization mapping such that rates by whichthe symbols are retrieved from the plurality of entropy decoders, aremade less dispersed. As to the binarizer 314 it is noted that same me beleft away if the syntax elements are already binary. Further, dependingon the type of decoder 322, the existence of the buffers 320 is notnecessary. Further, the buffers may be integrated within the decoders.

Termination of Finite Syntax Element Sequences

In an embodiment of the invention, the encoding and decoding is done fora finite set of syntax elements. Often a certain quantity of data suchas a still image, a frame or field of a video sequence, a slice of animage, a slice of a frame or a field of a video sequence, or a set ofsuccessive audio samples, etc. is coded. For finite sets of syntaxelements, in general, the partial bitstreams that are created at theencoder side have to be terminated, i.e., it has to be ensured that allsyntax elements can be decoded from the transmitted or stored partialbitstreams. After the last bin is inserted into the corresponding binbuffer 308, the bin encoder 310 has to ensure that a complete codewordis written to the partial bitstream 312. If the bin encoder 310represents an entropy encoder that implements a direct mapping of binsequences onto codewords, the bin sequence that is stored in the binbuffer after writing the last bin to the bin buffer might not representa bin sequence that is associated with a codeword (i.e., it mightrepresent a prefix of two or more bin sequences that are associated withcodewords). In such a case, any of the codewords associated with a binsequence that contains the bin sequence in the bin buffer as prefix hasto be written to the partial bitstream (the bin buffer has to beflushed). This could be done by inserting bins with a particular or anarbitrary value into the bin buffer until a codeword is written. In anembodiment of the invention, the bin encoder selects one of thecodewords with minimum length (in addition to the property that theassociated bin sequence must contain the bin sequence in the bin bufferas prefix). At the decoder side, the bin decoder 322 may decode morebins than necessitated for the last codeword in a partial bitstream;these bins are not requested by the bin buffer selector 318 and arediscarded and ignored. The decoding of the finite set of symbols iscontrolled by requests for decoded syntax elements; if no further syntaxelement is requested for a quantity of data, the decoding is terminated.

Transmission and Multiplexing of the Partial Bitstreams

The partial bitstreams 312 that are created by the encoder can betransmitted separately, or they can be multiplexed into a singlebitstream, or the codewords of the partial bitstreams can be interleavedin a single bitstream.

In an embodiment of the invention, each partial bitstream for a quantityof data is written to one data packet. The quantity of data can be anarbitrary set of syntax elements such as a still picture, a field orframe of a video sequence, a slice of a still picture, a slice of afield or frame of a video sequence, or a frame of audio samples, etc.

In another embodiment of the invention, two or more of the partialbitstreams for a quantity of data or all partial bitstreams for aquantity of data are multiplexed into one data packet. The structure ofa data packet that contains multiplexed partial bitstreams isillustrated in FIG. 9 .

The data packet 400 consists of a header and one partition for the dataof each partial bitstream (for the considered quantity of data). Theheader 400 of the data packet contains indications for the partitioningof the (remainder of the) data packet into segments of bitstream data402. Beside the indications for the partitioning, the header may containadditional information. In an embodiment of the invention, theindications for the partitioning of the data packet are the locations ofthe beginning of the data segments in units of bits or bytes ormultiples of bits or multiples of bytes. In an embodiment of theinvention, the locations of the beginning of the data segments are codedas absolute values in the header of the data packet, either relative tothe beginning of the data packet or relative to the end of the header orrelative to the beginning of the previous data packet. In a furtherembodiment of the invention, the locations of the beginning of the datasegments are differentially coded, i.e., only the difference between theactual beginning of a data segment and a prediction for the beginning ofthe data segment is coded. The prediction can be derived based onalready known or transmitted information such as the overall size of thedata packet, the size of the header, the number of data segments in thedata packet, the location of the beginning of preceding data segments.In an embodiment of the invention, the location of the beginning of thefirst data packet is not coded, but inferred based on the size of thedata packet header. At the decoder side, the transmitted partitionindications are used for deriving the beginning of the data segments.The data segments are then used as partial bitstreams and the datacontained in the data segments are fed into the corresponding bindecoders in sequential order.

There are several alternatives for multiplexing the partial bitstreamsinto a data packet. One alternative, which can reduce the necessitatedside information, in particular for cases in which the sizes of thepartial bitstreams are very similar, is illustrated in FIG. 10 . Thepayload of the data packet, i.e., the data packet 410 without its header411, is partitioned into segments 412 a predefined way. As an example,the data packet payload can be partitioned into segments of the samesize. Then each segment is associated with a partial bitstream or withthe first part of a partial bitstream 413. If a partial bitstream isgreater than the associated data segment, its remainder 414 is placedinto the unused space at the end of other data segments. This can bedone in a way that the remaining part of a bitstream is inserted inreverse order (starting from the end of the data segment), which reducesthe side information. The association of the remainders of the partialbitstreams to data segments and, when more than one remainder is addedto a data segment, the start point for one or more of the remaindershave to be signaled inside the bitstream, e.g. in the data packetheader.

Interleaving of Variable-Length Codewords

For some applications, the above described multiplexing of the partialbitstreams (for a quantity of syntax elements) in one data packet canhave the following disadvantages: On the one hand side, for small datapackets, the number of bits for the side information that isnecessitated for signaling the partitioning can become significantrelative to the actual data in the partial bitstreams, which finallyreduces the coding efficiency. On the other hand, the multiplexing maynot suitable for applications that necessitate a low delay (e.g. forvideo conferencing applications). With the described multiplexing, theencoder cannot start the transmission of a data packet before thepartial bitstreams have been completely created, since the locations ofthe beginning of the partitions are not known before. Furthermore, ingeneral, the decoder has to wait until it receives the beginning of thelast data segment before it can start the decoding of a data packet. Forapplications as video conferencing systems, these delays can add-up toan additional overall delay of the system of several video pictures (inparticular for bit rates that are close to the transmission bit rate andfor encoders/decoders that necessitate nearly the time interval betweentwo pictures for encoding/decoding a picture), which is critical forsuch applications. In order to overcome the disadvantages for certainapplications, the encoder of an embodiment of the invention can beconfigured in a way that the codewords that are generated by the two ormore bin encoders are interleaved into a single bitstream. The bitstreamwith the interleaved codewords can be directly send to the decoder (whenneglecting a small buffer delay, see below). At the decoder side, thetwo or more bin decoders read the codewords directly from the bitstreamin decoding order; the decoding can be started with the first receivedbit. In addition, no side information is required for signaling themultiplexing (or interleaving) of the partial bitstreams.

The basic structure of an encoder with codeword interleaving is shown inFIG. 11 . The bin encoders 310 don't write the codewords directly to thepartial bitstreams, but are connected with a single codeword buffer 329,from which codewords are written to the bitstream 34 in coding order.The bin encoders 310 send requests for one or more new codeword bufferentries 328 to the codeword buffer 329 and later send the codewords 30to the codeword buffer 329, which are stored in the reserved bufferentries. The (in general variable-length) codewords 31 of the codewordbuffer 329 are accessed by a codeword writer 32, which writes thecorresponding bits 33 to the produced bitstream 34. The codeword buffer329 operates as a first-in-first-out buffer; codeword entries that arereserved earlier are earlier written to the bitstream.

In a further generalization, multiple codeword buffers and partialbitstreams are possible, where the number of codeword buffers is lessthan the number of bin encoders. A bin encoder 310 reserves one or morecodewords in the codeword buffer 329, whereby the reservation of the oneor more codewords in the codeword buffer is triggered by certain eventsin the connected bin buffer 308. In an embodiment of the invention, thecodeword buffer 329 is operated in a way that the decoder caninstantaneously decode the bitstream. The coding order in which thecodewords are written to the bitstream is the same as the order in whichthe corresponding codewords are reserved in the codeword buffer. In anembodiment of the invention, each bin encoder 310 reserves one codeword,with the reservation being triggered by a certain event in the connectedbin buffer. In another embodiment of the invention, each bin encoder 310reserves more than one codeword, with the reservation being triggered bya certain event in the connected bin buffer. In further embodiment ofthe invention, the bin encoders 310 reserve a different amount ofcodewords, where the amount of codewords that are reserved by aparticular bin encoder can be dependent on the particular bin encoderand/or other properties of the particular bin encoder/bin buffer (suchas the associated probability measure, the number of already writtenbits, etc.).

In an embodiment of the invention, the codeword buffer is operated asfollows. If a new bin 307 is sent to a particular bin buffer 308 and thenumber of already stored bins in the bin buffer is zero and there iscurrently no codeword reserved in the codeword buffer for the binencoder that is connected with the particular bin buffer, the connectedbin encoder 310 sends a request to the codeword buffer, by which one ormore codeword entries are reserved in the codeword buffer 329 for theparticular bin encoder. The codeword entries can have a variable numberof bits; an upper threshold for the number of bits in a buffer entry isusually given by the maximum codeword size for the corresponding binencoder. The next codeword or the next codewords that are produced bythe bin encoder (for which the codeword entry or codeword entries havebeen reserved) are stored in the reserved entry or entries of thecodeword buffer. If all reserved buffer entries in the codeword bufferfor a particular bin encoder are filled with codewords and the next binis sent to the bin buffer that is connected with the particular binencoder, one or more new codewords are reserved in the codeword bufferfor the particular bin encoder, etc. The codeword buffer 329 representsa first-in-first-out buffer in a certain way. Buffer entries arereserved in sequential order. Codewords for which the correspondingbuffer entries have been reserved earlier are earlier written to thebitstream. The codeword writer 32 checks the status of the codewordbuffer 329, either continuously or after a codeword 30 is written to thecodeword buffer 329. If the first buffer entry contains a completecodeword (i.e., the buffer entry is not reserved, but includes acodeword), the corresponding codeword 31 and the corresponding bufferentry are removed from the codeword buffer 320 and the bits of thecodeword 33 are written to the bitstream. This process is repeated untilthe first buffer entry does not contain a codeword (i.e., it is reservedor free). At the end of the decoding process, i.e., if all syntaxelements of the considered quantity of data have been processed, thecodeword buffer must be flushed. For that flushing process, thefollowing is applied for each bin buffer/bin encoder as first step: Ifthe bin buffer does contain bins, a bin with a particular or anarbitrary value is added until the resulting bin sequence represents abin sequence that is associated with a codeword (as noted above, one wayof adding bins is to add such bin values that produce the shortestpossible codeword—or one of those—that is associated with a bin sequencethat contains the for the original content of the bin buffer as prefix),then the codeword is written to the next reserved buffer entry for thecorresponding bin encoder (and the corresponding) bin buffer is emptied.If more than one buffer entry has been reserved for one or more binencoders, the codeword buffer may still contain reserved codewordentries. In that case, these codeword entries are filled with arbitrary,but valid codewords for the corresponding bin encoders. In an embodimentof the invention, the shortest valid codeword or one of the shortestvalid codewords (if there are multiple) is inserted. Finally, allremaining codewords in the codeword buffer are written to the bitstream.

Two examples for the status of the codeword buffer are illustrated inFIG. 12 . In example (a), the codeword buffer contains 2 entries thatare filled with a codeword and 5 reserved entries. In addition, the nextfree buffer entry is marked. The first entry is filled with a codeword(i.e., the bin encoder 2 just wrote a codeword to a previously reservedentry). In the next step, this codeword will be removed from thecodeword buffer and written to the bitstream. Then, the first reservedcodeword for bin encoder 3 is the first buffer entry, but this entrycannot be removed from the codeword buffer, since it is only reserved,but no codeword has been written to this entry. In the example (b), thecodeword buffer contains 3 entries that are filled with a codeword and 4reserved entries. The first entry is marked as reserved and hence thecodeword writer cannot write a codeword to the bitstream. Although 3codewords are contained in the codeword buffer, the codeword writer hasto wait until a codeword is written to the first reserved buffer entryfor bin encoder 3. Note that the codewords must be written in the orderin which they were reserved, in order to be able to invert the processat the decoder side (see below).

The basic structure of a decoder with codeword interleaving is shown inFIG. 13 . The bin decoders 310 don't read the codewords directly fromseparate partial bitstreams, but are connected to a bit buffer 338, fromwhich the codewords 337 are read in coding order. It should be notedthat the bit buffer 338 is not necessarily required, since the codewordscould also be directly read from the bitstream. The bit buffer 338 ismainly included in the illustration for clearly separate differentaspects of the processing chain. The bits 339 of the bitstream 340 withinterleaved codewords are sequentially inserted into the bit buffer 338,which represents a first-in-first-out buffer. If a particular bindecoder 322 receives a request for one or more bin sequences 35, the bindecoder 322 reads one or more codewords 337 from the bit buffer 338 viarequests for bits 336. The decoder can instantaneously decode the syntaxelements. Note that the encoder (as described above) must ensure bysuitably operating the codeword buffer that the codewords are written inthe same order to the bitstream in which they are requested by the bindecoders. At the decoder, the entire decoding process is triggered byrequests for syntax elements. Parameters as the number of codewords thatare reserved at the encoder side by a particular bin encoder and thenumber of codewords that are read by the corresponding bin decoder mustbe the same.

In a further generalization, multiple codeword buffers and partialbitstreams are possible, where the number of bit buffers is less thanthe number of bin decoders. A bin decoder 322 reads one or morecodewords from the bit buffer 338 at one time instant, whereby thereading of the one or more codewords from the bit buffer is triggered bycertain events in the connected bin buffer 320. In an embodiment of theinvention, the decoder is operated in a way that one or more codewordsare read when a request for a bin 319 is sent to a particular bin buffer320 and the bin buffer doesn't contain any bins. But it is also possibleto trigger the reading of codewords by other events, e.g. if the numberof bins in the bin buffer is below a predefined threshold. In anembodiment of the invention, each bin decoder 322 reads one codeword,with the reading being triggered by a certain event in the connected binbuffer. In another embodiment of the invention, each bin decoder 322reads more than one codeword, with the reading being triggered by acertain event in the connected bin buffer. In a further embodiment ofthe invention, the bin decoders 322 read a different amount ofcodewords, where the amount of codewords that are read by a particularbin decoder can be dependent on the particular bin decoder and/or otherproperties of the particular bin decoder/bin buffer (such as theassociated probability measure, the number of already read bits, etc.).

In an embodiment of the invention, the reading of codewords from the bitbuffer is operated as follows. If a new bin request 319 is sent from thebin buffer selector 318 to a particular bin buffer 320 and the number ofbins in the bin buffer is zero, the connected bin decoder 322 reads oneor more codewords 337 from the bit buffer 338, via bit request 336 tothe bit buffer 338. The bin decoder 322 converts the read codewords 337into sequences of bins 321 and stores these bin sequences in theconnected bin buffer 320. As final response to the request for a bin319, the first inserted bin is removed from the bin buffer 320 and sentto the bin buffer selector 318. As response the further bin requests,the remaining bins in the bin buffer are removed until the bin buffer isempty. An additional bin request triggers the bin decoder to read one ormore new codewords from the bit buffer, etc. The bit buffer 338represents a first-in-first-out buffer of a predefined size and iscontinuously filled with bits 339 from the bitstream 340. In order toensure that the codewords are written to the bitstream in the same wayas they are requested by the decoding process, the codeword buffer atthe encoder side can be operated in the way described above.

Thus, each of the plurality of entropy decoders may be a variable lengthdecoder configured to map codewords of fixed lengths to symbol sequencesof variable lengths, and a codeword entry such as the output of thecodeword buffer 43 may be provided for receiving a single stream ofinterleaved codewords. The plurality of entropy decoders 22 may beconfigured to retrieve the codewords from the codeword entry in asequential order depending on an order in which the symbols of thesequence of symbols to be reconstructed as retrieved by the selector 18from the plurality of entropy decoders result in a new symbol sequenceto be mapped from a new codeword at the respective entropy decoders.

Interleaving of Variable-Length Codewords with a Low-Delay Constraint

The described codeword interleaving does not require that anypartitioning information is sent as side information. And since thecodewords are interleaved in the bitstream, the delay is in generalsmall. However, it is not guaranteed that a particular delay constraint(e.g. specified by a maximum number of bits that are stored in thecodeword buffer) is obeyed. Furthermore, the necessitated buffer sizefor the codeword buffer can theoretically become very large. Whenconsidering the example in FIG. 12(b), it might be possible that nofurther bins are send to bin buffer 3 and hence the bin encoder 3 willnot send any new codeword to the codeword buffer until the flushingprocess at the end of the data packet is applied. Then all codewords forbin encoders 1 and 2 would have to wait until the end of the datapacket, before they can be written to the bitstream. This drawback canbe circumvented by adding a further mechanism to the encoding process(and also to the decoding process as described later). The basic conceptof that additional mechanism is that if a measure related to the delayor an upper bound of the delay (see below) exceeds a specifiedthreshold, the first reserved buffer entry is filled by flushing thecorresponding bin buffer (using a similar mechanism as at the end of adata packet). By such a mechanism, the number of waiting buffer entriesis reduced until the associated delay measure is less than the specifiedthreshold. At the decoder side, the bins that have been inserted at theencoder side in order to obey the delay constraint must be discarded.For this discarding of bins basically the same mechanism as at theencoder side can be used. In the following two embodiments for such adelay control are described.

In one embodiment, the measure for the delay (or an upper bound of thedelay) is the number of active buffer entries in the codeword buffer,where the number of active buffer entries is the number of reservedbuffer entries plus the number of buffer entries that contain codewords.Note that the first buffer entry is a reserved buffer entry or a freebuffer entry, since if the first buffer entry contains a codeword, thiscodeword is written to the bitstream. If for example, the maximumallowed buffer delay (as determined by the application) is D bits andthe maximum codeword size for all bin encoders is L, a lower bound forthe maximum number of codewords that can be contained in the codewordbuffer without violating the delay constraint can be calculated byN=D/L. The delay measure D in bits is not required by the system, butthe maximum number of codewords N must be known to both encoder anddecoder. In an embodiment of the invention, the maximum number ofcodeword buffer entries N is fixed by the application. In anotherembodiment of the invention, the maximum number of codeword bufferentries N is signaled inside the bitstream, e.g., in the header of thedata packet (or slice header) or in a parameter set, which is includedin the bitstream. If a bin encoder 310 sends a request for thereservation of one or more new buffer entries to the codeword buffer329, the following process is executed before a new codeword bufferentry is reserved (i.e., it is executed multiple times if multiplecodeword buffer entries are reserved by one request): If the number ofcurrently active buffer entries plus 1 (taking into account the bufferentry that will be reserved next) is greater than the maximum number ofcodeword buffer entries N, the first buffer entry (which is reserved) isflushed by the process described in the following until the number ofcurrently active buffer entries plus 1 is less than or equal to themaximum number of codeword buffer entries N. The flushing of a reservedbuffer entry is similar to the flushing at the end of a data packet: Thebin encoder 310 that has reserved the corresponding first buffer entryis flushed by adding bins with particular or arbitrary values to theconnected bin buffer 308 until the resulting bin sequence represents abin sequence that is associated with a codeword, the codeword is thenwritten to the reserved buffer entry and it is finally added to thebitstream (while emptying the bin buffer and removing the previouslyreserved buffer entry). As mentioned above, one advantageous way foradding bins to the bin buffer is to add those bins that produce theshortest possible codeword. At the decoder side, a similar process isexecuted for discarding the bins that have been added to obey the delayconstraint. Therefore, the decoder maintains a counter C that counts thecodewords that have been read from the bit buffer (this counter can bemaintained in the bit buffer). This counter C is initialized (e.g. withzero) at the beginning of the decoding of a data packet and is increasedby one after a codeword is read. In addition, each bin decoder 322contains a counter Cx, which stores the value of the codeword counter Cbefore the last codeword was read by the corresponding bin decoder 322.I.e., when a particular bin decoder 322 reads a new codeword, itscounter Cx is set equal to C as a first step and then the codeword isread from the bit buffer. When a request for a bin 319 is sent to aparticular bin buffer 320 and the difference (C−Cx) between the overallcodeword counter C and the counter Cx of the connected bin decoder 322is greater than the maximum number of codeword buffer entries N, allbins that are currently stored in the particular bin buffer 320 arediscarded and ignored. Beside that additional step, the decoding isoperated as described above. If the bin buffer 320 to which a requestfor a bin 319 is sent is empty (either because all bins have alreadybeen removed or because the low-delay mechanism did discard all bins inthe first step after the bin request has been received), the connectedbin decoder 322 reads one or more new codewords from the bit buffer 338etc.

In another embodiment of the invention, the measure for the delay (or anupper bound of the delay) is the sum of the maximum codeword lengths forthe active buffer entries in the codeword buffer, where the maximumcodeword length for a particular buffer entry depends on the bin decodedthat is associated with that buffer entry. As illustration, the maximumcodeword lengths for the buffer entries are indicated in the examples in6. Note again that the first buffer entry is a reserved buffer entry ora free buffer entry, since if the first buffer entry contains acodeword, this codeword is written to the bitstream. Let the maximumallowed buffer delay (as determined by the application) be D bits. Thismaximum buffer delay D must be known to both encoder and decoder. In anembodiment of the invention, the maximum buffer delay D is fixed by theapplication. In another embodiment of the invention, the maximum bufferdelay D is signaled inside the bitstream, e.g., in the header of thedata packet (or slice header) or in a parameter set, which is includedin the bitstream. It can be signaled in units of bits, or bytes, or amultiple of bits, or a multiple of bytes. If a bin encoder 310 sends arequest for the reservation of one or more new buffer entries to thecodeword buffer 329, the following process is executed before a newcodeword buffer entry is reserved (i.e., it is executed multiple timesif multiple codeword buffer entries are reserved by one request).

If the sum of the maximum codeword lengths for all currently activebuffer entries plus the maximum codeword length for the buffer entrythat will be reserved is greater than the maximum buffer delay D, thefirst buffer entry (which is reserved) is flushed by the processdescribed above until the sum of the maximum codeword lengths for allactive buffer entries plus the maximum codeword length for the bufferentry that will be reserved is less than or equal to the maximum bufferdelay D. As an example, let's consider the example in FIG. 12(b). Thesum of the maximum codeword lengths for all currently active bufferentries is 29. Let's assume that the maximum buffer delay D is set equalto 32. If the next buffer entry is reserved by bin encoder 2 for whichthe maximum codeword length is equal to 3, the first buffer entry is notflushed, since 29+3 is not greater than 32. But if the next buffer entryis reserved by bin encoder 1 for which the maximum codeword length isequal to 7, the first buffer entry is flushed, since 29+7 is greaterthan 32. The flushing of the reserved buffer entry is done as describedabove (by adding bin with particular or arbitrary values to thecorresponding bin buffer).

At the decoder side, a similar process is executed for discarding thebins that have been added to obey the delay constraint. Therefore, thedecoder maintains a counter C that counts the maximum codeword lengthfor the codewords that have been read from the bit buffer (this countercan be maintained in the bit buffer). Note that the maximum codewordlengths that are associated with different bin decoders can bedifferent. The counter C is initialized (e.g. with zero) at thebeginning of the decoding of a data packet and it is increased after acodeword is read. This counter is not increased by the actual length ofthe read codewords, but by its maximum length. I.e., if a codeword isread by a particular bin decoder and the maximum codeword length that isassociated with the codeword table used by the particular bin decoder isLx (a different bin decoder can be associated with a different maximumcodeword length), the counter C is increased by Lx. In addition to theoverall counter C, each bin decoder 322 contains a counter Cx, whichstores the value of the codeword counter C before the last codeword wasread by the corresponding bin decoder 322. I.e., when a particular bindecoder 322 reads a new codeword, its counter Cx is set equal to C as afirst step and then the codeword is read from the bit buffer. When arequest for a bin 319 is sent to a particular bin buffer 320 and thedifference (C−Cx) between the overall counter C and the counter Cx ofthe connected bin decoder 322 is greater than the maximum buffer delayD, all bins that are currently stored in the particular bin buffer 320are discarded and ignored. Beside that additional step, the decoding isoperated as described above. If the bin buffer 320 to which a requestfor a bin 319 is sent is empty (either because all bins have alreadybeen removed or because the low-delay mechanism did discard all bins inthe first step after the bin request has been received), the connectedbin decoder 322 reads one or more new codewords from the bit buffer 338etc.

Thus, the plurality of entropy decoders 22 and the selector 18 may beconfigured to intermittently discard suffixes of symbol sequences so asto not participate in forming the sequence of symbols to bereconstructed. 29. The intermittently discarding may be performed atevents where a number of codewords having been retrieved from thecodeword entry by the plurality of entropy decoders between twoconsecutive codeword retrievals of a respective entropy decoder from thecodeword entry, fulfils a predetermined criterion. The plurality ofentropy encoders and the codeword buffer may, in turn, be configured tointermittently extend currently forwarded but not yet mapped symbols tovalid symbol sequences by don't-care symbols having the currentlyforwarded but not yet mapped symbols as prefix, map the thus extendedsymbol sequences into codewords, enter the thus obtained codewords intothe reserved codeword entries and flush the codeword entries. Theintermittently extending, entering and flushing may take place at eventswhere a number of reserved codeword entries plus a number of codewordentries having codewords entered therein fulfils a predeterminedcriterion. The predetermined criteria may take the maximum lengths ofcodewords of the plurality of encoder/decoder pairs into account.

For some architectures, the above described embodiment for the codewordinterleaving might result in a drawback in terms of the decodingcomplexity. As illustrated in FIG. 13 , all bin decoders 322 readcodewords (in the general case, variable-length codewords) from a singlebit buffer 338. The reading of the codewords cannot be done in parallel,since the codeword must be read in the correct order. That means, aparticular bin decoder must wait until other bin decoders finish thereading of codewords. And when the complexity of the reading of thevariable-length codewords is significant in relation to the remainder ofthe (partially parallelized) decoding process, this access of thevariable-length codewords can be a bottleneck for the entire decodingprocess. There are some variations of the described embodiments of theinvention that can be employed for reducing the complexity of the accessfrom the single bit buffer, a few of them will be described in thefollowing. In one embodiment of the invention, there exists a single setof codewords (representing for instance a redundancy-free prefix code)and the set of codewords that is used for each bin decoder 322 is asubset of the single codeword set. Note that different bin decoders 322can use different subsets of the single codeword set. Even if thecodeword sets that are used by some of the bin decoders 322 are thesame, their association with bin sequences is different for differentbin decoders 322. In a particular embodiment of the invention, the sameset of codewords is used for all bin decoders 322. If we have a singlecodeword set that includes the codeword sets for all bin decoders assubsets, the parsing of the codewords can be done outside the bindecoders, which can reduce the complexity of the codeword access. Theencoding process is not changed in relation to the above describedprocess. The modified decoding process is illustrated in FIG. 14 . Asingle codeword reader is fed with bits 346 from the bitstream 340 andparses the—in general variable-length—codewords. The read codewords 344are inserted into a codeword buffer 343, which represents afirst-in-first-out buffer. A bin decoder 322 sends a request for one ormore codewords 341 to the codeword buffer 343 and as response to thisrequest, the one or more codewords are removed from the codeword buffer(in sequential order) and send to the corresponding bin decoder 322.Note that with this embodiment of the invention, the potentially complexcodeword parsing can be done in a background process and it doesn't needto wait for the bin decoders. The bin decoders access already parsedcodewords, the potentially complex codeword parsing is no more part of arequest to the overall buffer. Instead already parsed codewords are sendto the bin decoders, which can also be implemented in a way that onlycodeword indices are send to the bin decoders.

Interleaving of Fixed-Length Bit Sequences

A further way of reducing the decoder complexity can be achieved whenthe bin decoders 322 don't read variable-length codewords from theglobal bit buffer 338, but instead they read fixed-length sequences ofbits from the global bit buffer 338 and add these fixed-length sequencesof bits to a local bit buffer, where each bin decoder 322 is connectedwith a separate local bit buffer. The variable-length codewords are thenread from the local bit buffer. Hence, the parsing of variable-lengthcodewords can be done in parallel, only the access of fixed-lengthsequences of bits has to be done in a synchronized way, but such anaccess of fixed-length sequences of bits is usually very fast, so thatthe overall decoding complexity can be reduced for some architectures.The fixed number of bins that are sent to a particular local bit buffercan be different for different local bit buffer and it can also varyover time, depending on certain parameters as events in the bin decoder,bin buffer, or bit buffer. However, the number of bits that are read bya particular access does not depend on the actual bits that are readduring the particular access, which is the important difference to thereading of variable-length codewords. The reading of the fixed-lengthsequences of bits is triggered by certain events in the bin buffers, bindecoders, or local bit buffers. As an example, it is possible to requestthe reading of a new fixed-length sequence of bits when the number ofbits that are present in a connected bit buffer falls below a predefinedthreshold, where different threshold values can be used for differentbit buffers. At the encoder, it has to be insured that the fixed-lengthsequences of bins are inserted in the same order into the bitstream, inwhich they are read from the bitstream at the decoder side. It is alsopossible to combine this interleaving of fixed-length sequences with alow-delay control similar to the ones explained above. In the following,an embodiment for the interleaving of fixed-length sequences of bits isdescribed.

FIG. 15 shows an illustration of the basic encoder structure for theembodiment of the invention that interleaves fixed-length sequences ofbits for two or more bin encoders. In contrast to the embodimentdepicted in FIG. 11 , the bin encoders 310 are not connected with asingle codeword buffer. Instead, each bin encoder 310 is connected witha separate bit buffer 348, which stores bits for the correspondingpartial bitstream. All bit buffers 348 are connected to a global bitbuffer 351. The global bit buffer 351 is connected to a bit writer 353,which removes the bits 352 in coding/decoding order from the global bitbuffer and writes the removed bits 354 to the bitstream 355. On acertain events in a particular bit buffer 348 or the connected binencoder 310 or bin buffer 308, the bit buffer 348 sends a request 349 tothe global bit buffer 351 by which a certain number of bits is reservedin the global bit buffer 351. The requests for the reservation offixed-length bit sequences 349 are processed in sequential order. Theglobal bit buffer 351 represents a first-in-first-out buffer in acertain way; bits that are reserved earlier are earlier written to thebitstream. It should be noted that different bit buffers 348 canreserved a different amount of bits, which can also vary over time basedon already coded symbols; but the number of bits that are reserved by aparticular request is known at the time at which the request is sent tothe global bit buffer.

In a particular embodiment of the invention, the bit buffers 348 and theglobal bit buffer 351 are operated as described in the following. Theamount of bits that is reserved by a particular bit buffer 348 isdenoted as Nx. This number of bits Nx can be different for different bitbuffers 348 and it can also vary over time. In an embodiment of theinvention, the number of bits Nx that are reserved by a particular bitbuffer 348 is fixed over time. The reservations for a fixed number Nx ofbits 349 are triggered based on the number of bits Mx in the bit buffers348, the number of bits Nx for the reservation requests, and theassociated maximum codeword length Lx. Note that each bin encoder 310can be associated with a different maximum codeword length Lx. If a bin307 is sent to a particular bin buffer 308, and the particular binbuffer 308 is empty, and not more than one sequence of Nx bits isreserved in the global bit buffer for the bit buffer 348 that isconnected with the particular bin buffer (via a bin encoder), and thedifference Nx−Mx between the number Nx of bits that are reserved by areservation request of the bit buffer 348 that is connected (via a binencoder) with the particular bin buffer 308 and the number of bits Mxthat are currently present in this bit buffer 348 is less than themaximum codeword length Lx that is associated with the corresponding binencoder 310, the connected bit buffer 349 sends a request 349 for thereservation of Nx bits to the global bit buffer 351. The global bitbuffer 351 reserves Nx bits for the particular bit buffer 348 andincreases its pointer for the next reservation. After the Nx bits havebeen reserved in the global bit buffer, the bin 307 is stored in binbuffer 308. If this single bin does already represent a bin sequencethat is associated with a codeword, the bin encoder 310 removes this binfrom the bin buffer 308 and writes the corresponding codeword 347 to theconnected bit buffer 348. Otherwise (this single bin does alreadyrepresent a bin sequence that is associated with a codeword), furtherbins 307 are accepted by the particular bin buffer 308 until the binbuffer 308 contains a bin sequence that is associated with a codeword.In this case, the connected bin encoder 310 removes the bin sequence 309from the bin buffer 308 and writes the corresponding codeword 347 to theconnected bit buffer 348. If the resulting number of bits Mx in the bitbuffer 348 is greater than or equal to the number of reserved bits Nx,the Nx bits that were first written to the bit buffer 348 are insertedinto the previously reserved space in the global bit buffer 351. For thenext bin 307 that is sent to the particular bin buffer 308, the sameprocess as specified above is executed; i.e., it is checked firstwhether a new number of Nx bits must be reserved in the global bitbuffer (if Nx−Mx is less than Lx) and then the bin is inserted into thebin buffer 308, etc. The bit writer writes the fixed-length bitsequences of the global bit buffer in the order in which they have beenreserved. If the first fixed-length entry in the global bit buffer 351contains a fixed-length bit sequence that has been actually inserted inthe global bit buffer (i.e., it is not only reserved), the bit writer353 removes the bits for this bit sequence 352 from the global bitbuffer 351 and writes the bits 354 to the bitstream. This process isrepeated until the first fixed-length entry in the global bit bufferrepresents a reserved or a free entry. If the first fixed-length entryin the global bit buffer represents a reserved entry, the bit writer 353waits until this entry is filled with actual bits before it writesfurther bits 354 to the bitstream 355.

At the end of a data packet, the bin buffers are flushed as describedabove. In addition, the bit buffers must be flushed by adding bits witha particular or an arbitrary value until all reserved buffer entries inthe global bit buffer are filled and written to the bitstream.

In FIG. 16 , two examples for the possible status of the global bitbuffer 351 are illustrated. In example (a), a case is illustrated inwhich different bit buffers/bin encoders reserve a different number ofbits. The global bit buffer contains 3 entries with actually writtenfixed-length bit sequences and 4 entries with reserved fixed-length bitsequences. The first fixed-length entry already contains actual bits(which must have been just inserted by bit buffer/bin encoder 2); thisentry (i.e., the corresponding 8 bits) can be removed and written to thebitstream. The next entry reserves 10 bits for bin encoder 3, but actualbits haven't been inserted yet. This entry cannot be written to thebitstream; it must be waited until the actual bits are inserted. In thesecond example (b), all bit buffers/bin encoders reserved the samenumber of bits (8 bits). The global bit buffer contains 4 reservationsfor 8 bit sequences and 3 actually written 8 bit sequences. The firstentry contains a reservation for 8 bits for bin encoder 3. Before anynew bits can be written to the bitstream, the bit writer has to waituntil bit buffer/bin encoder 3 writes the actual values of the 8 bitsinto this reserved entry.

FIG. 17 shows an illustration of the basic decoder structure for theembodiment of the invention that interleaves fixed-length sequences ofbits. In contrast to the embodiment depicted in FIG. 13 , the bindecoders 322 are not connected with a single bit buffer. Instead, eachbin decoder 322 is connected with a separate bit buffer 358, whichstores bits from the corresponding partial bitstream. All bit buffers358 are connected to a global bit buffer 361. The bits 362 from thebitstream 363 are inserted into the global bit buffer 361. On a certainevents in a particular bit buffer 358 or the connected bin decoder 322or bin buffer 320, the bit buffer 358 sends a request 359 to the globalbit buffer 361 by which a fixed-length sequence of bits 360 is removedfrom the global bit buffer 361 and inserted into the particular bitbuffer 358. The requests for the fixed-length bit sequences 359 areprocessed in sequential order. The global bit buffer 361 represents afirst-in-first-out buffer; bits that are earlier inserted into theglobal bit buffer are earlier removed. It should be noted that differentbit buffers 358 can request a different amount of bits, which can alsovary over time based on already decoded symbols; but the number of bitsthat are requested by a particular request is known at the time at whichthe request is sent to the global bit buffer. It should be noted thatthe global bit buffer 361 is not necessarily required, since thecodewords could also be directly read from the bitstream. The global bitbuffer 361 is mainly included in the illustration for clearly separatedifferent aspects of the processing chain.

In a particular embodiment of the invention, the bit buffers 358 and theglobal bit buffer 361 are operated as described in the following. Theamount of bits that is requested and read by a particular bit buffer 358is denoted as Nx, it is equal to the amount of bits that is written tothe global bit buffer by the corresponding bit buffer at the encoderside. This number of bits Nx can be different for different bit buffers358 and it can also vary over time. In an embodiment of the invention,the number of bits Nx that are requested and read by a particular bitbuffer 358 is fixed over time. The reading of a fixed number Nx of bits360 is triggered based on the number of bits Mx in the bit buffer 358and the associated maximum codeword length Lx. Note that each bindecoder 322 can be associated with a different maximum codeword lengthLx. If a request for a bin 319 is sent to a particular bin buffer 320,and the particular bin buffer 320 is empty, and the number Mx of bits inthe bit buffer 358 that is connected (via a bin decoder) with theparticular bin buffer 320 is less than the maximum codeword length Lxthat is associated with the corresponding bin decoder 322, the connectedbit buffer 358 sends a request 359 for a new sequences of Nx bits to theglobal bit buffer 361. As response to this request, the first Nx bitsare removed from to global bit buffer 361 and this sequence of Nx bits360 is sent to the bit buffer 358 from which the request was sent.Finally, this sequence of Nx bits is added to the corresponding bitbuffer 358. Then the next codeword 357 is read from this bit buffer, andthe connected bin decoder 322 inserts the associated bin sequence 321into the connected bin buffer 320. As final response to the originalrequest for a bin 319, the first bin is removed from the bin buffer 320and this decoded bin 325 is sent to the bin buffer selector 318. Whenthe next bin request 319 is sent to the particular bin buffer 320 andthe bin buffer is not empty, the next bit is removed from the bin buffer320. If the bin buffer is empty but the number Mx of bits in theconnected bit buffer 358 is greater than or equal to the associatedmaximum codeword length Lx, the next codeword is read from the bitbuffer and a new bin sequence is inserted in the bin buffer, from whichthe first bit is removed and sent to the bin buffer selector. If the binbuffer is empty and the number Mx of bits in the connected bit buffer358 is less than the associated maximum codeword length Lx, the nextsequence of Nx bits is read from the global bit buffer 361 and insertedinto the connected local bit buffer 358, the next codeword is read fromthe bit buffer, a new bin sequence is inserted in the bin buffer, andthe first bin of the sequence is removed and sent to the bin bufferselector. This process is repeated until all syntax elements aredecoded.

At the end of a data packet, more bins and/or bits than necessitated fordecoding the requested syntax elements might be inserted into the binbuffer and/or bit buffer. The remaining bins in the bin buffer and theremaining bits in the bit buffer are discarded and ignored.

Interleaving of Fixed-Length Bit Sequences with a Low-Delay Constraint

The described embodiment for an entropy encoder and decoder withinterleaving of fixed-length bit sequences can also be combined with thescheme for controlling the encoder buffer delay, which is describedabove. The basic concept is the same as in the embodiment with delaycontrol described above. If a measure related to the delay or an upperbound of the delay (see below) exceeds a specified threshold, the firstreserved buffer entry is filled by flushing the corresponding bin buffer(using a similar mechanism as at the end of a data packet) andpotentially writing additional bits for filling all bits of the reservedfixed-length buffer entry. By such a mechanism, the number of waitingbuffer entries is reduced until the associated delay measure is lessthan the specified threshold. At the decoder side, the bins and bitsthat have been inserted at the encoder side in order to obey the delayconstraint must be discarded. For this discarding of bins and bitsbasically the same mechanism as at the encoder side can be used.

In an embodiment of the invention, the measure for the delay (or anupper bound of the delay) is the number of bits in the active bufferentries in the global bit buffer, where the number of active bufferentries is the number of reserved fixed-length buffer entries plus thenumber of fixed-length buffer entries that contain already written bits.Note that the first buffer entry is a reserved fixed-length buffer entryor a free buffer entry, since if the first buffer entry contains writtenbits, these bits are written to the bitstream. Let the maximum allowedbuffer delay (as determined by the application) be D bits. This maximumbuffer delay D must be known to both encoder and decoder. In anembodiment of the invention, the maximum buffer delay D is fixed by theapplication. In another embodiment of the invention, the maximum bufferdelay D is signaled inside the bitstream, e.g., in the header of thedata packet (or slice header) or in a parameter set, which is includedin the bitstream. It can be signaled in units of bits, or bytes, or amultiple of bits, or a multiple of bytes. If a bin encoder 310 sends arequest for the reservation of a new fixed-length bit sequence to theglobal bit buffer 351, the following process is executed before a newfixed-length buffer entry is reserved.

If the number of bits in the active buffer entries in the global bitbuffer plus the number of bits that will be reserved by the currentreservation request is greater than the maximum buffer delay D, thefirst buffer entry (which is reserved) is flushed by the processdescribed in the following until the number of bits in the active bufferentries in the global bit buffer plus the number of bits that will bereserved by the current reservation request is less than or equal to themaximum buffer delay D. The flushing of a reserved fixed-length bufferentry is similar to the flushing at the end of a data packet: The binencoder 310 that is connected with the bit buffer 348 that has reservedthe corresponding first buffer entry is flushed by adding bins withparticular or arbitrary values to the connected bin buffer 308 until theresulting bin sequence represents a bin sequence that is associated witha codeword, the codeword is then inserted into the corresponding bitbuffer 348. As mentioned above, one advantageous way for adding bins tothe bin buffer is to add those bins that produce the shortest possiblecodeword. If, after the writing of the codeword to the connected bitbuffer and a potential insertion of a fixed-length bit sequence into theglobal bit buffer, there are still bits in the bit buffer (i.e., thewritten codeword did not completely fill the reserved fixed-lengthsequence of bits), further bits with particular or arbitrary values areadded to the bit buffer until all bits are removed from the bit bufferand written to the reserved buffer entry. Finally, at the end of thisprocess, the completed buffer entry (the first fixed-length entry in theglobal bit buffer) is removed from the global bit buffer and written tothe bitstream.

At the decoder side, a similar process is executed for discarding thebins and bits that have been added to obey the delay constraint.Therefore, the decoder maintains a counter C that counts the bits thathave been read from the global bit buffer (this counter can bemaintained in the global bit buffer). The counter C is initialized (e.g.with zero) at the beginning of the decoding of a data packet and it isincreased after a fixed-length sequence of is read. If a fixed-lengthsequence of Nx bits is read from the global bit buffer 361, the counterC is increased by Nx. In addition to the overall counter C, each bitbuffer 358 contains a counter Cx, which stores the value of the bitcounter C before the last fixed-length bit sequence was read into thecorresponding bit buffer 358. When a particular bit buffer 358 reads anew fixed-length bit sequence, its counter Cx is set equal to C as afirst step and then the fixed-length bit sequence is read from theglobal bit buffer 361. When a request for a bin 319 is sent to aparticular bin buffer 320 and the difference (C−Cx) between the overallcounter C and the counter Cx of the connected bit buffer 358 is greaterthan the maximum buffer delay D, all bins that are currently stored inthe particular bin buffer 320 and all bits that are stored in theconnected bit buffer 358 are discarded and ignored. Beside thatadditional step, the decoding is operated as described above. If the binbuffer 320 to which a request for a bin 319 is sent is empty (eitherbecause all bins have already been removed or because the low-delaymechanism did discard all bins in the first step after the bin requesthas been received), the connected bin decoder 322 attempts to read a newcodeword from the connected bit buffer 358. If the number of bits in thebit buffer 358 is less than the maximum codeword length, a newfixed-length bit sequence is read from the global bit buffer 361, beforethe codeword is read, etc.

After having described embodiments according to which the evenpreviously coding is used for compressing video data, is described as aneven further embodiment for implementing embodiments of the presentinvention which renders the implementation especially effective in termsof a good trade-off between compression rate on the one hand and look-uptable and computation overhead on the other hand. In particular, thefollowing embodiments enable the use of computationally less complexvariable length codes in order to entropy-code the individuallybitstreams, and effectively cover portions of the probability estimate.In the embodiments described below, the symbols are of binary nature andthe VLC codes presented below effectively cover the probability estimaterepresented by, for example, R_(LPS), extending within [0;0.5].

In particular, the embodiments outlined below describe possibleimplementations for the individual entropy coders 310 and decoders 322in FIG. 7 to 17 , respectively. They are suitable for coding of bins,i.e. binary symbols, as they occur in image or video compressionapplications. Accordingly, these embodiments are also applicable toimage or video coding where such binary symbols are split-up into theone or more streams of bins 307 to be encoded and bitstreams 324 to bedecoded, respectively, where each such bin stream can be considered as arealization of a Bernoulli process. The embodiments described below useone or more of the below-explained various so-calledvariable-to-variable-codes (v2v-codes) to encode the bin streams. Av2v-code can be considered as two prefix-free codes with the same numberof code words. A primary, and a secondary prefix-free code. Each codeword of the primary prefix-free code is associated with one code word ofthe secondary prefix-free code. In accordance with the below-outlinedembodiments, at least some of the encoders 310 and decoders 322, operateas follows: To encode a particular sequence of bins 307, whenever a codeword of the primary prefix-free code is read from buffer 308, thecorresponding code-word of the secondary prefix-free code is written tothe bit stream 312. The same procedure is used to decode such a bitstream 324, but with primary and secondary prefix-free codeinterchanged. That is, to decode a bitstream 324, whenever a code wordof the secondary prefix-free code is read from the respective bit stream324, the corresponding code-word of the primary prefix-free code iswritten to buffer 320.

Advantageously, the codes described below do not necessitate look-uptables. The codes are implementable in form of finite state machines.The v2v-codes presented here, can be generated by simple constructionrules such that there is no need to store large tables for the codewords. Instead, a simple algorithm can be used to carry out encoding ordecoding. Three construction rules are described below where two of themcan be parameterized. They cover different or even disjoint portions ofthe afore-mentioned probability interval and are, accordingly,specifically advantageous if used together, such as all three codes inparallel (each for different ones of the en/decoders 11 and 22), or twoof them. With the construction rules described below, it is possible todesign a set of v2v-codes, such that for Bernoulli processes witharbitrary probability p, one of the codes performs well in terms ofexcess code length.

As stated above, the encoding and decoding of the streams 312 and 324respectively, can either be performed independently for each stream orin an interleaved manner. This, however, is not specific to thepresented classes of v2v-codes and therefore, only the encoding anddecoding of a particular codeword is described for each of the threeconstruction rules in the following. However, it is emphasized, that allof the above embodiments concerning the interleaving solutions are alsocombinable with the presently described codes or en- and decoders 310and 322, respectively.

Construction Rule 1: ‘Unary Bin Pipe’ Codes or En-/Decoders 310 and 322

Unary bin pipe codes (PIPE=probability interval partitioning entropy)are a special version of the so-called ‘bin pipe’ codes, i.e. codessuitable for coding of any of the individual bitstreams 12 and 24, eachtransferring data of a binary symbol statistics belonging to a certainprobability sub-interval of the afore-mentioned probability range[0;0.5]. The construction of bin pipe codes is described first. A binpipe code can be constructed from any prefix-free code with at leastthree code words. To form a v2v-code, it uses the prefix-free code asprimary and secondary code, but with two code words of the secondaryprefix-free code interchanged. This means that except for two codewords, the bins are written to the bit stream unchanged. With thistechnique, only one prefix-free code needs to be stored along with theinformation, which two code words are interchanged and thus, memoryconsumption is reduced. Note, that it only makes sense to interchangecode words of different length since otherwise, the bit stream wouldhave the same length as the bin stream (neglecting effects that canoccur at the end of the bin stream).

Due to this construction rule, an outstanding property of the bin pipecodes is, that if primary and secondary prefix-free code areinterchanged (while the mapping of the code words is retained), theresulting v2v-code is identical to the original v2v-code. Therefore, theencoding algorithm and decoding algorithm are identical for bin-pipecodes.

A unary bin pipe code is constructed from a special prefix-free code.This special prefix-free code is constructed as follows. First, aprefix-free code consisting of n unary code words is generated startingwith ‘01’, ‘001’, ‘0001’, . . . until n code words are produced. n isthe parameter for the unary bin pipe code. From the longest code word,the trailing 1 is removed. This corresponds to a truncated unary code(but without the code word ‘0’). Then, n−1 unary code words aregenerated starting with ‘10’, ‘110’, ‘1110’, . . . until n−1 code wordsare produced. From the longest of these code words, the trailing 0 isremoved. The union set of these two prefix-free codes are used as inputto generate the unary bin pipe code. The two code words that areinterchanged are the one only consisting of 0s and the one onlyconsisting of 1s.

-   -   Example for n=4:

Nr Primary Secondary 1 0000 111 2 0001 0001 3 001 001 4 01 01 5 10 10 6110 110 7 111 0000Construction rule 2: ‘Unary to rice’ codes and Unary to riceen−/decoders 10 and 22:

Unary to rice codes use a truncated unary code as primary code. I.e.unary code words are generated starting with ‘1’, ‘01’, ‘001’, . . .until 2^(n)+1 code words are generated and from the longest code word,the trailing 1 is removed. n is the parameter of the unary to rice code.The secondary prefix-free code is constructed from the code words of theprimary prefix-free code as follows. To the primary code word onlyconsisting of 0s, the code word ‘1’ is assigned. All other code wordsconsist of the concatenation of the code word ‘0’ with the n-bit binaryrepresentation of the number of 0s of the corresponding code word of theprimary prefix-free code.

-   -   Example for n=3:

Nr Primary Secondary 1 1 0000 2 01 0001 3 001 0010 4 0001 0011 5 000010100 6 000001 0101 7 0000001 0110 8 00000001 0111 9 00000000 1 NOTE:that this is identical to mapping an infinite unary code to a rice codewith rice parameter 2^(n).

Construction Rule 3: ‘Three Bin’ Code

The three bin code is given as:

Nr Primary Secondary 1 000 0 2 001 100 3 010 101 4 100 110 5 110 11100 6101 11101 7 011 11110 8 111 11111

It has the property, that the primary code (symbol sequences) is offixed length (three bins) and the code words are sorted by ascendingnumbers of Is.

An efficient implementation of three bin code is described next. Anencoder and decoder for the three bin code can be implemented withoutstoring tables in the following way.

In the encoder (any of 10), three bins are read from the bin stream(i.e. 7). If these three bins contain exactly one 1, the code word ‘1’is written to the bit stream followed by two bins consisting of thebinary representation of the position of the 1 (starting from right with00). If the three bins contain exactly one 0, the code word ‘111’s iswritten to the bit stream followed by two bins consisting of the binaryrepresentation of the position of the 0 (starting from the right with00). The remaining code words ‘000’ and ‘111’ are mapped to ‘0’ and‘11111’, respectively.

In the decoder (any of 22), one bin or bit is read from the respectivebitstream 24. If it equals ‘0’, the code word ‘000’ is decoded to thebin stream 21. If it equals ‘1’, two more bins are read from the bitstream 24. If these two bits do not equal ‘11’, they are interpreted asthe binary representation of a number and two 0s and one 1 is decoded tothe bit stream such that the position of the 1 is determined by thenumber. If the two bits equal ‘11’, two more bits are read andinterpreted as binary representation of a number. If this number issmaller than 3, two is and one 0 are decoded and the number determinesthe position of the 0. If it equals 3, ‘111’ is decoded to the binstream.

An efficient implementation of unary bin pipe codes is described next.An encoder and decoder for unary bin pipe codes can be efficientlyimplemented by using a counter. Due to the structure of bin pipe codes,encoding and decoding of bin pipe codes is easy to implement:

In the encoder (any of 10), if the first bin of a code word equals ‘0’,bins are processed until a ‘1’ occurs or until n 0s are read (includingthe first ‘0’ of the code word). If a ‘1’ occurred, the read bins arewritten to the bit stream unchanged. Otherwise (i.e. n 0s were read),n−1 is are written to the bit stream. If the first bin of the code wordequals ‘1’, bins are processed until a ‘0’ occurs or until n−1 is areread (including the first ‘1’ of the code word). If a ‘0’ occurred, theread bins are written to the bit stream unchanged. Otherwise (i.e. n−1is were read), n 0s are written to the bit stream.

In the decoder (any of 322), the same algorithm is used as for theencoder, since this is the same for bin pipe codes as described above.

An efficient implementation of unary to rice codes is described next. Anencoder and decoder for unary to rice codes can be efficientlyimplemented by using a counter as will be described now.

In the encoder (any of 310), bins are read from the bin stream (i.e. 7)until a 1 occurs or until 2^(n) 0s are read. The number of 0s iscounted. If the counted number equals 2^(n), the code word ‘1’ iswritten to the bit stream. Otherwise, ‘0’ is written, followed by thebinary representation of the counted number, written with n bits.

In the decoder (any of 322), one bit is read. If it equals ‘1’, 2^(n) 0sare decoded to the bin string. If it equals ‘0’, n more bits are readand interpreted as binary representation of a number. This number of 0sis decoded to the bin stream, followed by a ‘1’.

In other words, the just-described embodiments describe an encoder forencoding a sequence of symbols 303, comprising an assigner 316configured to assign a number of parameters 305 to each symbol of thesequence of symbols based on information contained within previoussymbols of the sequence of symbols; a plurality of entropy encoders 310each of which is configured to convert the symbols 307 forwarded to therespective entropy encoder 310 into a respective bitstream 312; and aselector 6 configured to forward each symbol 303 to a selected one ofthe plurality of entropy encoders 10, the selection depending on thenumber of parameters 305 assigned to the respective symbol 303.According to the just-outlined embodiments, at least a first subset ofthe entropy encoders may be a variable length encoder configured to mapsymbol sequences of variable lengths within the stream of symbols 307 tocodewords of variable lengths to be inserted in bitstream 312,respectively, with each of the entropy coders 310 of the first subsetusing a bijective mapping rule according to which code words of aprimary prefix-free code with (2n−1)≥3 code words are mapped to codewords of a secondary prefix-free code which is identical to the primaryprefix code such that all but two of the code words of the primaryprefix-free code are mapped to identical code words of the secondaryprefix-free code while the two code words of the primary and secondaryprefix-free codes have different lengths and are mapped onto each otherin an interchanged manner, wherein the entropy encoders may usedifferent n so as to covers different portions of an interval of theabove-mentioned probability interval. The first prefix-free code may beconstructed such that the codewords of the first prefix-free code are(a,b)₂, (a,a,b)₃, . . . , (a, . . . , a,b)_(n), (a, . . . , a)_(n),(b,a)₂, (b,b,a)₃, . . . , (b, . . . , b,a)_(n-1), (b, . . . , b)_(n-1),and the two codewords mapped onto each other in the interchanged mannerare (a, . . . , a)_(n) and (b, . . . , b)_(n-1) with b≠a and a,b∈{0,1}.However, alternatives are feasible.

In other words, each of a first subset of entropy encoders may beconfigured to, in converting the symbols forwarded to the respectiveentropy encoder into the respective bitstream, examine a first symbolforwarded to the respective entropy encoder, to determine as to whether(1) the first symbol equals a ∈{0,1}, in which case the respectiveentropy encoder is configured to examine the following symbols forwardedto the respective entropy encoder to determine as to whether (1.1) bwith b≠a and b∈{0,1} occurs within the next n−1 symbols following thefirst symbol, in which case the respective entropy encoder is configuredto write a codeword to the respective bitstream, which equals the firstsymbol followed by the following symbols forwarded to the respectiveentropy encoder, up to the symbol b; (1.2) no b occurs within the nextn−1 symbols following the first symbol, in which case the respectiveentropy encoder is configured to write a codeword to the respectivebitstream, which equals (b, . . . , b)_(n-1); or (2) the first symbolequals b, in which case the respective entropy encoder is configured toexamine the following symbols forwarded to the respective entropyencoder to determine as to whether (2.1) a occurs within the next n−2symbols following the first symbol, in which case the respective entropyencoder is configured to write a codeword to the respective bitstream,which equals the first symbol followed by the following symbolsforwarded to the respective entropy encoder up to the symbol a; or (2.2)no a occurs within the next n−2 symbols following the first symbol, inwhich case the respective entropy encoder is configured to write acodeword to the respective bitstream, which equals (a, . . . , a)_(n).

Additionally or alternatively, a second subset of the entropy encoders10 may be a variable length encoder configured to map symbol sequencesof variable lengths to codewords of fixed lengths, respectively, witheach of the entropy coders of the second subset using a bijectivemapping rule according to which code words of a primary truncated unarycode with 2^(n)+1 code words of the type {(a), (ba), (bba), . . . , (b .. . ba), (bb . . . b)} with b≠a and a,b∈{0,1} are mapped to code wordsof a secondary prefix-free code such that the codeword (bb . . . b) ofthe primary truncated unary code is mapped onto codeword (c) of thesecondary prefix-free code and all other codewords {(a), (ba), (bba), .. . , (b . . . ba)} of the primary truncated unary code are mapped ontocodewords having (d) with c≠d and c,d∈{0,1} as a prefix and a n-bit wordas suffix, wherein the entropy encoders use different n. Each of thesecond subset of entropy encoders may be configured such that the n-bitword is an n-bit representation of the number of b's in the respectivecodeword of the primary truncated unary code. However, alternatives arefeasible.

Again, from the perspective of the mode of operation of the respectiveencoder 10, each of the second subset of entropy encoders may beconfigured to, in converting the symbols forwarded to the respectiveentropy encoder into the respective bitstream, count a number of b's ina sequence of symbols forwarded to the respective entropy encoder, untilan a occurs, or until the number of the sequence of symbols forwarded tothe respective entropy encoder reaches 2^(n) with all 2^(n) symbols ofthe sequence being b, and (1) if the number of b's equals 2^(n), write cwith c∈{0,1} as codeword of a secondary prefix-free code to therespective bitstream, and (2) if the number of b's is lower than 2^(n),write a codeword of the secondary prefix-free code to the respectivebitstream, which has (d) with c≠d and d∈{0,1} as prefix and a n-bit worddetermined depending on the number of b's as suffix.

Also additionally or alternatively, a predetermined one of the entropyencoders 10 may be a variable length encoder configured to map symbolsequences of fixed lengths to codewords of variable lengths,respectively, the predetermined entropy coder using a bijective mappingrule according to which 23 code words of length 3 of a primary code aremapped to code words of a secondary prefix-free code such that thecodeword (aaa)₃ of the primary code with a ∈{0,1} is mapped ontocodeword (c) with c∈{0,1}, all three codewords of the primary codehaving exactly one b with b≠a and b∈{0,1} are mapped onto codewordshaving (d) with c≠d and d∈{0,1} as a prefix and a respective first 2-bitword out of a first set of 2-bit words as a suffix, all three codewordsof the primary code having exactly one a are mapped onto codewordshaving (d) as a prefix and a concatenation of a first 2-bit word notbeing an element of the first set and a second 2-bit word out of asecond set of 2-bit words, as a suffix, and wherein the codeword (bbb)₃is mapped onto a codeword having (d) as a prefix and a concatenation ofthe first 2-bit word not being an element of the first set and a second2-bit word not being an element of the second set, as a suffix. Thefirst 2-bit word of the codewords of the primary code having exactly oneb may be a 2-bit representation of a position of the b in the respectivecodeword of the primary code, and the second 2-bit word of the codewordsof the primary code having exactly one a may be a 2-bit representationof a position of the a in the respective codeword of the primary code.However, alternatives are feasible.

Again, the predetermined one of the entropy encoders may be configuredto, in converting the symbols forwarded to the predetermined entropyencoder into the respective bitstream, examine the symbols to thepredetermined entropy encoder in triplets as to whether (1) the tripletconsists of a's, in which case the predetermined entropy encoder isconfigured to write the codeword (c) to the respective bitstream, (2)the triplet exactly comprises one b, in which case the predeterminedentropy encoder is configured to write a codeword having (d) as a prefixand a 2-bit representation of a position of the b in the triplet as asuffix, to the respective bitstream; (3) the triplet exactly comprisesone a, in which case the predetermined entropy encoder is configured towrite a codeword having (d) as a prefix and a concatenation of the first2-bit word not being an element of the first set and a 2-bitrepresentation of a position of the a in the triplet as a suffix, to therespective bitstream; or (4) the triplet consists of b's, in which casethe predetermined entropy encoder is configured to write a codewordhaving (d) as a prefix and a concatenation of the first 2-bit word notbeing an element of the first set and the first 2-bit word not being anelement of the second set as a suffix, to the respective bitstream.

Regarding the decoding side, just-described embodiments disclose adecoder for reconstructing a sequence of symbols 326, comprising aplurality of entropy decoders 322, each of which is configured toconvert a respective bitstream 324 into symbols 321; an assigner 316configured to assign a number of parameters to each symbol 326 of asequence of symbols to be reconstructed based on information containedwithin previously reconstructed symbols of the sequence of symbols; anda selector 318 configured to retrieve each symbol 325 of the sequence ofsymbols to be reconstructed from a selected one of the plurality ofentropy decoders, the selection depending on the number of parametersdefined to the respective symbol. According to the just-describedembodiments at least a first subset of the entropy decoders 322 arevariable length decoders configured to map codewords of variable lengthsto symbol sequences of variable lengths, respectively, with each of theentropy decoders 22 of the first subset using a bijective mapping ruleaccording to which code words of a primary prefix-free code with(2n−1)≥3 code words are mapped to code words of a secondary prefix-freecode which is identical to the primary prefix code such that all but twoof the code words of the primary prefix-free code are mapped toidentical code words of the secondary prefix-free code while the twocode words of the primary and secondary prefix-free codes have differentlengths and are mapped onto each other in an interchanged manner,wherein the entropy encoders use different n. The first prefix-free codemay be constructed such that the codewords of the first prefix-free codeare (a,b)₂, (a,a,b)₃, (a, . . . , a,b)_(n), (a, . . . , a)_(n), (b,a)₂,(b,b,a)₃, . . . , (b, . . . , b,a)_(n-1), (b, . . . , b)_(n-1), and thetwo codewords mapped onto each other in the interchanged manner may be(a, . . . , a)_(n) and (b, . . . , b)_(n-1) with b≠a and a,b∈{0,1}.However, alternatives are feasible.

Each of the first subset of entropy encoders may be configured to, inconverting the respective bitstream into the symbols, examine a firstbit of the respective bitstream, to determine as to whether (1) thefirst bit equals a 0 {0,1}, in which case the respective entropy encoderis configured to examine the following bits of the respective bitstreamto determine as to whether (1.1) b with b≠a and b 0 {0,1} occurs withinthe next n−1 bits following the first bit, in which case the respectiveentropy decoder is configured to reconstruct a symbol sequence, whichequals the first bit followed by the following bits of the respectivebitstream, up to the bit b; or (1.2) no b occurs within the next n−1bits following the first bit, in which case the respective entropydecoder is configured to reconstruct a symbol sequence, which equals (b,. . . , b)_(n-1); or (2) the first bit equals b, in which case therespective entropy decoder is configured to examine the following bitsof the respective bitstream to determine as to whether (2.1) a occurswithin the next n−2 bits following the first bit, in which case therespective entropy decoder is configured to reconstruct a symbolsequence, which equals the first bit followed by the following bits ofthe respective bitstream up to the symbol a; or (2.2) no a occurs withinthe next n−2 bits following the first bit, in which case the respectiveentropy decoder is configured to reconstruct a symbol sequence, whichequals (a, . . . , a)_(n).

Additionally or alternatively, at least a second subset of the entropydecoders 322 may be a variable length decoder configured to mapcodewords of fixed lengths to symbol sequences of variable lengths,respectively, with each of the entropy decoders of the second subsetusing a bijective mapping rule according to which code words of asecondary prefix-free code are mapped onto code words of a primarytruncated unary code with 2^(n)+1 code words of the type ((a), (ba),(bba), . . . , (b . . . ba), (bb . . . b)) with b≠a and a,b∈{0,1} suchthat codeword (c) of the secondary prefix-free code is mapped onto thecodeword (bb . . . b) of the primary truncated unary code and codewordshaving (d) with c≠d and c,d∈{0,1} as a prefix and a n-bit word assuffix, are mapped to a respective one of the other codewords {(a),(ba), (bba), . . . , (b . . . ba)} of the primary truncated unary code,wherein the entropy decoders use different n. Each of the second subsetof entropy decoders may be configured such that the n-bit word is ann-bit representation of the number of b's in the respective codeword ofthe primary truncated unary code. However, alternatives are feasible.

Each of a second subset of entropy decoders may be a variable lengthdecoder configured to map codewords of fixed lengths to symbol sequencesof variable lengths, respectively, and configured to, in converting thebitstream of the respective entropy decoder into the symbols, examine afirst bit of the respective bitstream to determine as to whether (1)same equals c with c∈{0,1}, in which case the respective entropy decoderis configured to reconstruct a symbol sequence which equals (bb . . .b)₂ ^(n) with b E {0,1}; or (2) same equals d with c≠d and c,d∈{0,1}, inwhich case the respective entropy decoder is configured to determine an-bit word from n further bits of the respective bitstream, followingthe first bit, and reconstruct a symbol sequence therefrom which is ofthe type {(a), (ba), (bba), . . . , (b . . . ba), (bb . . . b)} with b:a and b∈{0,1} with the number of b's depending on the n-bit word.

Additionally or alternatively, a predetermined one of the entropydecoders 322 may be a variable length decoders configured to mapcodewords of variable lengths to symbol sequences of fixed lengths,respectively, the predetermined entropy decoder using a bijectivemapping rule according to which code words of a secondary prefix-freecode are mapped to 23 code words of length 3 of a primary code such thatcodeword (c) with c∈{0,1} is mapped to the codeword (aaa)₃ of theprimary code with a ∈{0,1}, codewords having (d) with c≠d and d∈{0,1} asa prefix and a respective first 2-bit word out of a first set of three2-bit words as a suffix are mapped onto all three codewords of theprimary code having exactly one b with b≠a and b∈{0,1}, codewords having(d) as a prefix and a concatenation of a first 2-bit word not being anelement of the first set and a second 2-bit word out of a second set ofthree 2-bit words, as a suffix are mapped onto all three codewords ofthe primary code having exactly one a, and a codeword having (d) as aprefix and a concatenation of the first 2-bit word not being an elementof the first set and a second 2-bit word not being an element of thesecond set, as a suffix is mapped onto the codeword (bbb)₃. The first2-bit word of the codewords of the primary code having exactly one b maybe a 2-bit representation of a position of the b in the respectivecodeword of the primary code, and the second 2-bit word of the codewordsof the primary code having exactly one a may be a 2-bit representationof a position of the a in the respective codeword of the primary code.However, alternatives are feasible.

The predetermined one of the entropy decoders may be a variable lengthdecoder configured to map codewords of variable lengths to symbolsequences of three symbols each, respectively, and configured to, inconverting the bitstream of the respective entropy decoder into thesymbols, examine the first bit of the respective bitstream to determineas to whether (1) the first bit of the respective bitstream equals cwith c∈{0,1}, in which case the predetermined entropy decoder isconfigured to reconstruct a symbol sequence which equals (aaa)₃ with a 0{0,1}, or (2) the first bit of the respective bitstream equals d withc≠d and d ∈{0,1}, in which case the predetermined entropy decoder isconfigured to determine a first 2-bit word from 2 further bits of therespective bitstream, following the first bit, and examine the first2-bit word to determine as to whether (2.1) the first 2-bit word is noelement of a first set of three 2-bit words, in which case thepredetermined entropy decoder is configured to reconstruct a symbolsequence which has exactly one b with b≠a and b 0 {0,1}, with theposition of b in the respective symbol sequence depending on the first2-bit word, or (2.2) the first 2-bit word is element of the first set,in which case the predetermined entropy decoder is configured todetermine a second 2-bit word from 2 further bits of the respectivebitstream, following the two bits from which the first 2-bit word hasbeen determined, and examine the second 2-bit word to determine as towhether (3.1) the second 2-bit word is no element of a second set ofthree 2-bit words, in which case the predetermined entropy decoder isconfigured to reconstruct a symbol sequence which has exactly one a,with the position of a in the respective symbol sequence depending onthe second 2-bit word, or (3.2) the second 2-bit word is element of asecond set of three 2-bit words, in which case the predetermined entropydecoder is configured to reconstruct a symbol sequence which equals(bbb)₃.

Now, after having described the general concept of a video codingscheme, embodiments of the present invention are described with respectto the above embodiments. In other words, the embodiments outlined belowmay be implemented by use of the above schemes, and vice versa, theabove coding schemes may be implemented using und exploiting theembodiments outlined below.

In the above embodiments described with respect to FIG. 7 to 9 , theentropy encoder and decoders of FIG. 1 to 6 , were implemented inaccordance with an PIPE concept. One special embodiment used arithmeticsingle-probability state an/decoders 310 and 322. As will be describedbelow, in accordance with an alternative embodiment, entities 306-310and the corresponding entities 318 to 322 may be replaced by a commonarithmetic encoding engine, which manages merely one common state R andL and encodes all symbols into one common bitstream, thereby giving-upthe advantageous aspects of the present PIPE concept regarding parallelprocessing, but avoiding the necessity of interleaving the partialbitstreams as further discussed below. In doing so, the number ofprobability states by which the context's probabilities are estimated byupdate (table look-up), may be higher than the number of probabilitystates by which the probability interval sub-division is performed. Thatis, analogously to quantizing the probability interval width valuebefore indexing into the table Rtab, also the probability state indexmay be quantized. The above description for a possible implementationfor the single en/decoders 310 and 322 may, thus, be extended for anexample of an implementation of the entropy en/decoders 318-322/306-310as context-adaptive binary arithmetic en/decoding engines:

To be more precise, in accordance with an embodiment, the entropyencoder attached to the output of parameter assigner which acts as acontext assigner, here) operates in the following way:

-   -   0. The assigner 304 forwards the bin value along with the        probability parameter. The probability is pState_current[bin].    -   1. Thus, the entropy encoding engine receives: 1) valLPS, 2) the        bin and 3) the probability distribution estimate        pState_current[bin]. pState_current[bin] may have more states        than the number of distinguishable probability state indices of        Rtab. If so, pState_current[bin] may be quantized such as, for        example, by disregarding m LSBs with m being greater than or        equal to 1 and advantageously 2 or 3 so as to obtain an p_state,        i.e the index which is then used to access the table Rtab. The        quantization may, however, be left away, i.e. p_state may be        pState_current[bin].    -   2. Then, a quantization of R is performed (As mentioned above:        either one R (and corresponding L with one common bitstream) is        used/managed for all distinguishable values of p_state, or one R        (and corresponding L with associated partial bitstream per R/L        pair) per distinguishable value of p_state which latter case        would correspond to having one bin encoder 310 per such value)        -   q_index=Qtab[R>>q] (or some other form of quantization)    -   3. Then, a determination of R_(LPS) and R is performed:        -   R_(LPS)=Rtab[p_state][q_index]; Rtab has stored therein            pre-calculated values for p[p_state] Q[q_index]        -   R=R−R_(LPS) [that is, R is preliminarily pre-updated as if            “bin” was MPS]    -   4. Calculation of the new partial interval:        -   if (bin=1−valMPS) then

L¬L+R

R¬R_(LPS)

-   -   5. Renormalization of L and R, writing bits,

Analogously, the entropy decoder attached to the output of parameterassigner (which acts as a context assigner, here) operates in thefollowing way:

-   -   0. The assigner 304 forwards the bin value along with the        probability parameter. The probability is pState_current[bin].    -   1. Thus, the entropy decoding engine receives the request for a        bin along with: 1) valLPS, and 2) the probability distribution        estimate pState_current[bin]. pState_current[bin] may have more        states than the number of distinguishable probability state        indices of Rtab. If so, pState_current[bin] may be quantized        such as, for example, by disregarding m LSBs with m being        greater than or equal to 1 and advantageously 2 or 3 so as to        obtain an p_state, i.e the index which is then used to access        the table Rtab. The quantization may, however, be left away,        i.e. p_state may be pState_current[bin].    -   2. Then, a quantization of R is performed (As mentioned above:        either one R (and corresponding V with one common bitstream) is        used/managed for all distinguishable values of p_state, or one R        (and corresponding V with associated partial bitstream per R/L        pair) per distinguishable value of p_state which latter case        would correspond to having one bin encoder 310 per such value)        -   q_index=Qtab[R>>q] (or some other form of quantization)    -   3. Then, a determination of R_(LPS) and R is performed:        -   R_(LPS)=Rtab[p_state][q_index]; Rtab has stored therein            pre-calculated values for p[p_state]·Q[q_index]        -   R=R−R_(LPS) [that is, R is preliminarily pre-updated as if            “bin” was MPS]    -   4. Determination of bin depending on the position of the partial        interval:

if (V ³ R) then  bin ¬ 1 − valMPS (bin is decoded as LPS;  bin bufferselector 18 will obtain the  actual bin value by use of this bininformation and valMPS)  V ¬ V − R  R ¬ R_(LPS) else  bin ¬ valMPS (binis decoded as MPS; the   actual bin value is obtained by use of this  bin information and valMPS)

-   -   5. Renormalization of R, reading out one bit and updating V,

As described above, the assigner 4 assigns pState_current[bin] to eachbin. The association may be done based on a context selection. That is,assigner 4 may select a context using an context index ctxIdx which, inturn, has a respective pState_current associated therewith. Aprobability update may be performed each time, a probabilitypState_current[bin] has been applied to a current bin. An update of theprobability state pState_current[bin] is performed depending on thevalue of the coded bit:

if (bit = 1 − valMPS) then  pState_current □ Next_State_LPS[pState_current]  if (pState_current = 0) then valMPS □ 1 − valMPS else pState_current □ Next_State_MPS [pState_current]

If more than one context is provided, the adaptation is donecontext-wise, i.e. pState_current[ctxIdx] is used for coding and thenupdated using the current bin value (encoded or decoded, respectively).

As will be outlined in more detail below, in accordance with embodimentsdescribed now, the encoder and decoder may optionally be implemented tooperate in different modes, namely Low complexity (LC), and Highefficiency (HE) mode. This is illustrated regarding PIPE coding in thefollowing (then mentioning LC and HE PIPE modes), but the description ofthe complexity scalability details is easily transferable to otherimplementations of the entropy encoding/decoding engines such as theembodiment of using one common context-adaptive arithmetic en/decoder.

In accordance with the embodiments outlined below, both entropy codingmodes may share

-   -   the same syntax and semantics (for the syntax element sequence        301 and 327, respectively)    -   the same binarization schemes for all syntax elements (as        currently specified for CABAC) (i.e. binarizers may operate        irrespective of the mode activated)    -   the usage of the same PIPE codes (i.e. bin en/decoders may        operate irrespective of the mode activated)    -   the usage of 8 bit probability model initialization values        (instead of 16 bit initialization values as currently specified        for CABAC)

Generally speaking, LC-PIPE differs from HE-PIPE in the processingcomplexity, such as the complexity of selecting the PIPE path 312 foreach bin.

For example, the LC mode may operate under the following constraints:For each bin (binIdx), there may be exactly one probability model, i.e.,one ctxIdx. That is, no context selection/adaptation may be provided inLC PIPE. Specific syntax elements such as those used for residual codingmay, hover, coded using contexts, as further outlined below. Moreover,all probability models may be non-adaptive, i.e., all models may beinitialized at the beginning of each slice with appropriate modelprobabilities (depending on the choice of slice type and slice QP) andmay be kept fixed throughout processing of the slice. For example, only8 different model probabilities corresponding to 8 different PIPE codes310/322 may be supported, both for context modelling and coding.Specific syntax elements for residual coding, i.e.,significance_coeff_flag and coeff_abs_level_greaterX (with X=1,2), thesemantics of which are outlied in more detail below, may be assigned toprobability models such that (at least) groups of, for example, 4 syntaxelements are encoded/decoded with the same model probability. Comparedto CAVLC, the LC-PIPE mode achieves roughly the same R-D performance andthe same throughput.

HE-PIPE may be configured to be conceptually similar to CABAC of H.264with the following differences: Binary arithmetic coding (BAC) isreplaced by PIPE coding (same as in the LC-PIPE case). Each probabilitymodel, i.e., each ctxIdx, may be represented by a pipeIdx and arefineIdx, where pipeIdx with values in the range from 0 . . . 7represents the model probability of the 8 different PIPE codes. Thischange affects only the internal representation of states, not thebehavior of the state machine (i.e., probability estimation) itself. Aswill be outlined in more detail below, the initialization of probabilitymodels may use 8 bit initialization values as stated above. Backwardscanning of syntax elements coeff_abs_level_greaterX (with X=1, 2),coeff_abs_level_minus3, and coeff_sign_flag (the semantics of which willget clear from the below discussion) may be performed along the samescanning path as the forward scan (used in, for example, thesignificance map coding). Context derivation for coding ofcoeff_abs_level_greaterX (with X=1, 2) may also be simplified. Comparedto CABAC, the proposed HE-PIPE achieves roughly the same R-D performanceat a better throughput.

It is easy to see that the just-mentioned modes are readily generated byrendering, for example, the afore-mentioned context-adaptive binaryarithmetic en/decoding engine such that same operates in differentmodes.

Thus, in accordance with an embodiment in accordance with a first aspectof the present invention, a decoder for decoding a data stream may beconstructed as shown in FIG. 18. The decoder is for decoding adatastream 401, such as interleaved bitstream 340, into which mediadata, such as video data, is coded. The decoder comprises a mode switch400 configured to activate the low-complexity mode or the highefficiency mode depending on the data stream 401. To this end, the datastream 401 may comprise a syntax element such as a binary syntaxelement, having a binary value of 1 in case of the low-complexity modebeing the one to be activated, and having a binary value of 0 in case ofthe high efficiency mode being the one to be activated. Obviously, theassociation between binary value and coding mode could be switched, anda non-binary syntax element having more than two possible values couldbe used as well. As the actual selection between both modes is not yetclear before the reception of the respective syntax element, this syntaxelement may be contained within some leading header of the datastream401 encoded, for example, with a fixed probability estimate orprobability model or being written into the datastream 401 as it is,i.e., using a bypass mode.

Further, the decoder of FIG. 18 comprises a plurality of entropydecoders 322 each of which is configured to convert codewords in thedatastream 401 to partial sequences 321 of symbols. As described above,a de-interleaver 404 may be connected between inputs of entropy decoders322 on the one hand and the input of the decoder of FIG. 18 where thedatastream 401 is applied, on the other hand. Further, as alreadydescribed above, each of the entropy decoders 322 may be associated witha respective probability interval, the probability intervals of thevarious entropy decoders together covering the whole probabilityinterval from 0 to 1- or 0 to 0.5 in case of the entropy decoders 322dealing with MPS and LPS rather than absolute symbol values. Detailsregarding this issue have been described above. Later on, it is assumedthat the number of decoders 322 is 8 with a PIPE index being assigned toeach decoder, but any other number is also feasible. Further, one ofthese coders, in the following this is exemplarily the one havingpipe_id 0, is optimized for bins having equi-probable statistics, i.e.their bin value assumes 1 and 0 equally probably. This, decoder maymerely pass on the bins. The respective encoder 310 operates the same.Even any bin manipulation depending on the value of the most probablebin value, valMPS, by the selectors 402 and 502, respectively, may beleft away. In other words, the entropy of the respective partial streamis already optimal.

Further, the decoder of FIG. 18 comprises a selector 402 configured toretrieve each symbol of a sequence 326 of symbols from a selected one ofthe plurality of entropy decoders 322. As mentioned above, selector 402may be split-up into a parameter assigner 316 and a selector 318. Ade-symbolizer 314 is configured to de-symbolize the sequence 326 ofsymbols in order to obtain a sequence 327 of syntax elements. Areconstructor 404 is configured to reconstruct the media data 405 basedon the sequence of syntax elements 327. The selector 402 is configuredto perform the selection depending on the activated one of the lowcomplexity mode and the high-efficiency mode as it is indicated by arrow406.

As already noted above, the reconstructor 404 may be the part of apredictive block-based video decoder operating on a fixed syntax andsemantics of syntax elements, i.e., fixed relative to the mode selectionby mode switch 400. That is, the construction of the reconstructor 404does not suffer from the mode switchability. To be more precise, thereconstructor 404 does not increase the implementation overhead due tothe mode switchability offered by mode switch 400 und at least thefunctionality with regard to the residual data and the prediction dataremains the same irrespective of the mode selected by switch 400. Thesame applies, however, with regard to the entropy decoders 322. Allthese decoders 322 are reused in both modes and, accordingly, there isno additional implementation overhead although the decoder of FIG. 18 iscompatible with both modes, the low-complexity and high-efficiencymodes.

As a side aspect it should be noted that the decoder of FIG. 18 is notonly able to operate on self-contained datastreams either in the onemode or the other mode. Rather, the decoder of FIG. 18 as well as thedatastream 401 could be configured such that switching between bothmodes would even be possible during one piece of media data such asduring a video or some audio piece, in order to, for example, controlthe coding complexity at the decoding side depending on external orenvironmental conditions such as a battery status or the like with usinga feedback channel from decoder to encoder in order to accordinglylocked-loop control the mode selection.

Thus, the decoder of FIG. 18 operates similarly in both cases, in caseof the LC mode being selected or the HE mode being selected. Thereconstructor 404 performs the reconstruction using the syntax elementsand requests the current syntax element of a predetermined syntaxelement type by processing or obeying some syntax structureprescription. The de-symbolizer 314 requests a number of bins in orderto yield a valid binarization for the syntax element requested by thereconstructor 404. Obviously, in case of a binary alphabet, thebinarization performed by de-symbolizer 314 reduces down to merelypassing the respective bin/symbol 326 to reconstructor 404 as the binarysyntax element currently requested.

The selector 402, however, acts independently on the mode selected bymode switch 400. The mode of operation of selector 402 tends to be morecomplex in case of the high efficiency mode, and less complex in case ofthe low-complexity mode. Moreover, the following discussion will showthat the mode of operation of selector 402 in the less-complex mode alsotends to reduce the rate at which selector 402 changes the selectionamong the entropy decoders 322 in retrieving the consecutive symbolsfrom the entropy decoders 322. In other words, in the low-complexitymode, there is an increased probability that immediately consecutivesymbols are retrieved from the same entropy decoder among the pluralityof entropy decoders 322. This, in turn, allows for a faster retrieval ofthe symbols from the entropy decoders 322. In the high-efficiency mode,in turn, the mode of operation of the selector 402 tends to lead to aselection among the entropy decoders 322 where the probability intervalassociated with the respective selected entropy decoder 322 more closelyfits to the actual symbol statistics of the symbol currently retrievedby selector 402, thereby yielding a better compression ratio at theencoding side when generating the respective data stream in accordancewith the high-efficiency mode.

For example, the different behavior of the selector 402 in both modes,may be realized as follows. For example, the selector 402 may beconfigured to perform, for a predetermined symbol, the selection amongthe plurality of entropy decoders 322 depending on previously retrievedsymbols of the sequence 326 of symbols in case of the high-efficiencymode being activated and independent from any previously retrievedsymbols of the sequence of symbols in case of the low-complexity modebeing activated. The dependency on previously retrieved symbols of thesequence 326 of symbols may result from a context adaptivity and/or aprobability adaptivity. Both adaptivities may be switched off during lowcomplexity mode in selector 402.

In accordance with a further embodiment, the datastream 401 may bestructured into consecutive portions such as slices, frames, group ofpictures, frame sequences or the like, and each symbol of the sequenceof symbols may be associated with a respective one of a plurality ofsymbol types. In this case, the selector 402 may be configured to vary,for symbols of a predetermined symbol type within a current portion, theselection depending on previously retrieved symbols of the sequence ofsymbols of the predetermined symbol type within the current portion incase of the high-efficiency mode being activated, and leave theselection constant within the current portion in case of thelow-complexity mode being activated. That is, selector 402 may beallowed to change the selection among the entropy decoders 322 for thepredetermined symbol type, but these changes are restricted to occurbetween transitions between consecutive portions. By this measure,evaluations of actual symbol statistics are restricted to seldomoccurring time instances while coding complexity is reduced within themajority of the time.

Further, each symbol of the sequence 326 of symbols may be associatedwith a respective one of a plurality of symbol types, and the selector402 may be configured to, for a predetermined symbol of a predeterminedsymbol type, select one of a plurality of contexts depending onpreviously retrieved symbols of the sequence 326 of symbols and performthe selection among the entropy decoders 322 depending on a probabilitymodel associated with a selected context along with updating theprobability model associated with a selected context depending on thepredetermined symbol in case of the high-efficiency mode beingactivated, and perform selecting the one of the plurality of contextdepending on the previously retrieved symbols of the sequence 326 ofsymbols and perform the selection among the entropy decoders 322depending on the probability model associated with the selected contextalong with leaving the probability model associated with the selectedcontext constant in case of the low-complexity mode being activated.That is, selector 402 may use context adaptivity with respect to acertain syntax element type in both modes, while suppressing probabilityadaptation in case of the LC mode.

Alternatively, instead of completely suppressing the probabilityadaptation, selector 402 may merely reduce an update rate of theprobability adaptation of the LC mode relative to the HE mode.

Further, possible LC-pipe-specific aspects, i.e., aspects of the LCmode, could be described as follows in other words. In particular,non-adaptive probability models could be used in the LC mode. Anon-adaptive probability model can either have a hardcoded, i.e.,overall constant probability or its probability is kept fixed throughoutprocessing of a slice only and thus can be set dependent on slice typeand QP, i.e., the quantization parameter which is, for example, signaledwithin the datastream 401 for each slice. By assuming that successivebins assigned to the same context follow a fixed probability model, itis possible to decode several of those bins in one step as they areencoded using the same pipe code, i.e., using the same entropy decoder,and a probability update after each decoded bin is omitted. Omittingprobability updates saves operations during the encoding and decodingprocess and, thus, also leads to complexity reductions and a significantsimplification in hardware design.

The non-adaptive constraint may be eased for all or some selectedprobability models in such a way that probability updates are allowedafter a certain number of bins have been encoded/decoded using thismodel. An appropriate update interval allows a probability adaptationwhile having the ability to decode several bins at once.

In the following, a more detailed description of possible common andcomplexity-scalable aspects of LC-pipe and HE-pipe is presented. Inparticular, in the following, aspects are described which may be usedfor LC-pipe mode and HE-pipe mode in the same way or in acomplexity-scalable manner. Complexity-scalable means that the LC-caseis derived from the HE-case by removing particular parts or by replacingthem with something less complex. However, before proceeding therewith,it should be mentioned that the embodiment of FIG. 18 is easilytransferable onto the above-mentioned context-adaptive binary arithmeticen/decoding embodiment: selector 402 and entropy decoders 322 wouldcondense into an entropy decoder 608 which would receive the datastream401 directly and select the context for a bin currently to be derivedfrom the datastream. This is especially true for context adaptivityand/or probability adaptivity. Both functionalities/adaptivities may beswitched off, or designed more relaxed, during low complexity mode. Thatis, the entropy decoding engine 608 could generally be configured toretrieve each symbol of a sequence 326 of symbols by entropy decodingfrom the data stream 401 using a selected one of a plurality of entropydecoding schemes, and could, for example, be configured such that eachof the plurality of entropy decoding schemes involves arithmeticdecoding of the symbols the respective entropy decoding scheme has beenselected for, with the plurality of entropy decoding schemes differingfrom each other in using a different probability estimate in thearithmetic decoding. As described in connection with the above-outlinedCABAC-concept, the entropy decoding engine could be configured such thatthe plurality of entropy decoding schemes perform their probabilitysub-division on a common probability interval, i.e. one common bitstreamrather than partial bitstreams. In other words, and more generallyspeaking, the entropy decoder 608 could be configured to derive a numberof bins 326 of the binarizations from the data stream 401 using binaryentropy decoding by selecting a context among different contexts andupdating probability states associated with the different contexts,dependent on previously decoded portions of the data stream 401. To bemore precise, as described above the entropy decoder 608 may beconfigured to derive the number of bins 326 of the binarizations fromthe data stream 401 using binary entropy decoding such as theabove-mentioned CABAC scheme, or binary PIPE decoding, i.e. using theconstruction involving several parallel operating entropy decoders 322along with a respective selector/assigner. As far as the contextselection is concerned, the dependency thereof on the previously decodedportions of the data stream 401, may be embodied as outlined above. Thatis, the entropy decoder 608 may be configured to perform the contextselection for a bin currently to be derived depending on a bin positionof the bin currently to the derived within the binarization to which thebin currently to be derived belongs, a syntax element type of a syntaxelement, the integer value of which is obtained by debinarizing thebinarization to which the bin currently to be derived belongs, or one ormore bins previously derived from the data stream 401 or the integervalue of a syntax element previously debinarized. For example, thecontext selected may differ between the first and second bin of thebinarization of a certain syntax element. Moreover, different groups ofcontexts may be provided for different syntax element types such astransform coefficient levels, motion vector differences, coding modeparameters and the like. As far as the probability state update isconcerned, entropy decoder 608 may be configured to perform same, for abin currently derived, by transitioning from a current probability stateassociated with the context selected for the bin currently derived to anew probability state depending on the bin currently derived. Asdescribed above, the entropy decoder 409 may, for example, access atable entry using the current state and the value of the bin currentlyderived with the accessed table entry revealing the new probabilitystate. See above tables Next_State_LPS and Next_State_MPS the tablelook-up with respect to which is performed by the entropy decoder 608 inaddition to the other steps 0 to 5 listed above. In the descriptionabove, the probability state was sometimes denoted aspState_current[bin]. As also described above, the entropy decoder 608may be configured to binary arithmetic decode a bin currently to bederived by quantizing a current probability interval bit value (R)representing a current probability interval to obtain a probabilityinterval index q_index and performing an interval subdivision byindexing a table entry among table entries (Rtab) using the probabilityinterval index and a probability state index p_state which depends on acurrent probability state associated with the context selected for thebin currently to be derived, to obtain the subdivision of the currentprobability interval into two partial intervals. As described, theentropy decoder 608 may use an 8 bit representation for the currentprobability interval width value R. For quantizing the currentprobability width value, the entropy decoder 608 may, for example,grab-out two or three most significant bits of the 8 bit representation.Entropy decoder 608 may then perform the selection among the two partialintervals based on an offset state value from an interior of the currentprobability interval, update the probability interval width value and anoffset state value, and infer a value of the bin currently to bederived, using the selected partial interval and perform arenormalization of the updated probability width value and the offsetstate value, namely V in the above description, including a continuationof reading bits from the data stream 401. As described above, theselection among the two partial intervals based on the offset statevalue V may involve a comparison between R and V while the update of theprobability interval width value and the offset state value may dependon the value of the bin currently to be derived.

In implementing the embodiment of FIG. 18 with PIPE, the pipe entropycoding stage involving the entropy decoders 322 could use eightsystematic variable-to-variable-codes, i.e., each entropy decoder 322could be of a v2v type which has been described above. The PIPE codingconcept using systematic v2v-codes is simplified by restricting thenumber of v2v-codes. In case of a context-adaptive binary arithmeticdecoder, same could manage the same probability states for the differentcontexts and use same—or a quantoized version thereof—for theprobability sub-division. The mapping of CABAC or probability modelstates, i.e. the sates used for probability update, to PIPE ids orprobability indices for look-up into Rtab may be as depicted in Table A.

TABLE A Mapping of CABAC states to PIPE indices CABAC state PIPE index 0 0  1  2  3 1  4  5  6  7  8  9 10 2 11 12 13 14 15 3 16 17 18 19 2021 22 4 23 24 25 26 27 28 29 30 31 32 5 33 34 35 36 37 38 39 40 41 42 4344 45 46 6 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 7

This modified coding scheme may be used as a basis for thecomplexity-scalable video coding approach. When performing probabilitymode adaptation, the selector 402 or context-adaptive binary arithmeticdecoder, respectively, would select the PIPE decoder 322, i.e. derivethe pipe index, to be used, and the probability index into Rtab,respectively, based on the probability state index—here exemplarilyranging from 0 to 62—associated with the currently to be decodedsymbol—such as via a context—using the mapping shown in table A, andwould update this probability state index depending on the currentlydecoded symbol using, for example, specific table walk transition valuespointing to the next probability state index to be visited in case of anMPS and a LPS, respectively. In case of LC mode, the latter update couldbe left away. Even the mapping could be left away in case of globallyfixed probability models.

However, an arbitrary entropy coding setup could be used and thetechniques in this document can also be used with minor adaptations.

The above description of FIG. 12 rather generally referred to syntaxelements and syntax element types. In the following, a complexityconfigurable coding of transform coefficient levels is described.

The currently envisaged coding technique for transform coefficientlevels is specified in the current Test Model (HM) of the HighEfficiency Video Coding (HEVC) project for CABAC. First, the lastsignificant scan position is coded with two syntax elements,last_signifcant_pos_x and last_signifcant_pos_y. The syntax elementlast_signifcant_pos_x specifies the column of the last significant scanposition and the second syntax element last_signifcant_pos_y specifiesthe row of the last significant scan position.

After that, the significance map, which specifies the location ofabsolute transform coefficient levels greater than zero, is coded usinga forward scan order. The so-called significance map scan order is amapping from the two dimensional position inside a transform block ontoa one dimensional vector and can be different depending on theprediction mode and transform block size. In the state-of-the-art, threedifferent scan orders are used, namely the zigzag scan, the horizontalscan and the vertical scan. For each scan position except the last scanposition, which is already identified as significant by specifying thelast scan position, the binary syntax element coeff_signficant_flag iscoded.

Next, after the coding of the significance map, the syntax elementscoeff_abs_greater1, coeff_abs_greater2, and coeff_abs_minus3 representthe remaining absolute level and coeff_sign_flag represents the signinformation are coded. For the coding of the remaining absolutetransform levels and the sign, the transform block is divided into 4×4sub-blocks and the positions inside such a sub-block formed a subset.The subsets are scanned in forward zigzag scan order and each subset iscoded successively. It means that after all remaining information of theabsolute transform level of one 4×4 sub-block or subset and the signinformation are coded, the next subset in the forward zigzag order isprocessed. For the subset itself, the reverse zigzag scan order is used.In the first coding stage of a subset, the binary syntax elementcoeff_abs_greater1 specifying if the absolute transform coefficientlevel is greater than one is coded for all significant scan position ofthe subset. Next, after the scan order is reset and begins from thefirst scan position of the subset again, for all scan positions withabsolute levels greater than one, e.g. coeff_abs_greater1 is equal to 1,the binary syntax element coeff_abs_greater2 is coded specifying if theabsolute transform level for the specific scan position is greater thantwo or not. Then, after resetting the scan order again, for all scanpositions with absolute transform level greater than two thenon-negative integer valued syntax element coeff_abs_minus3 specifyingthe remaining value of the absolute transform level is coded. In thelast step, again after resetting the scan order, the syntax elementcoeff_sign_flag is coded in the bypass mode, e.g. with a probabilitymodel equal to 0.5 is coded. The reason for the partitioning intosubsets is the better context modelling results in higher codingefficiency, which is described in the following. Note that there is adependency between the syntax elements. It is also possible to code thesyntax elements as in H.264/AVC. In that case, for a scan position, thecoeff_abs_greater1 is coded directly after the coding acoeff_signficant_flag equal to one and coeff_abs_greater2 is codeddirectly after the coding of a coeff_abs_greater1 and so on. However,this interleaved mode of coding the syntax elements is inefficient for ahardware implementation. Therefore, a separation by coding each syntaxelement completely for a transform block or for a subset is described.

The context modelling for each syntax element related to the coding ofthe absolute transform levels are as follows. The context modelselection for coeff_significant_flag employs a local template. Itspecifies a region around the current scan position and covers alreadycoded scan positions. A context model is derived forcoeff_signficant_flag by evaluating the number of already codedsignificant scan positions lying in the local template.

For the first significant scan position of a subset, the second contextmodel of the context model set related to coeff_abs_greater1 is selectedin order to code the first coeff_abs_greater1 syntax element of thesubset. If a just coded coeff_abs_greater1 syntax element is equal to 1,the first context model is selected for all remaining coding stage ofthe syntax element coeff_abs_greater1 inside of the subset. Otherwise,the next context model of the context model set for the syntax elementcoeff_abs_greater1 is selected, while the maximum context model islimited to five.

For coeff_abs_greater2, the first context model is selected and isincremented after the coding of each coeff_abs_greater2 syntax element,while the maximum context is also limited to five. Note that differentcontext models are used for bins related to different syntax elements,e.g. different context model sets are used for the coeff_abs_greater1and coeff_abs_greater2 syntax elements. The remaining non-negativeinteger value of the absolute transform level is represented by thecoeff_abs_minus3 syntax element. It is coded for each scan position withcoeff_abs_greater2 equal to 1. For coeff_abs_minus3, a combination ofparameterized Rice and 0th order Exp-Golomb variable length codes (VLC)is used as binarization and all bins of the resulting bin string fromthe binarization are coded in a low complexity bypass mode with a fixedprobability of 0.5. The Rice part of the binarization forabs_coeff_minus3 necessitates a control parameter, which is zero in thebeginning of each subset and is updated adaptively after coding of eachcoeff_abs_minus3 syntax element. The new value of the control parameteris depending on the just coded remaining value represented bycoeff_abs_minus3. Note that the context modelling rule is reset for eachsubsets so that the second context model for coeff_abs_greater1 and thefirst context model for coeff_abs_greater2 and the binarization controlparameter for coeff_abs_minus3 is zero for the first position inside asubset, where the respective syntax element is coded. For thecoeff_abs_greater1 and coeff_abs_greater2 syntax elements, an additionalcontext modelling stage depends on the statistics of previous codedsubsets are made. This additional step derives a context offset orcontext set. The offset or context set for both syntax elements isderived as follows. At the beginning of each subset, a context set oroffset is selected depending on the numbers of absolute levels greaterthan one in the previous subset containing at least one significantposition. If there is no such subset, the set is 5 (for the first subsetcontaining significant scan positions of the transform block). For 4×4transform blocks, where the only one subset covers the whole transformblock, the context set is set to zero. Otherwise, the context set can beranged from 1 to 5. After coding the remaining absolute transformlevels, the sign information is coded in a bypass mode, e.g. with acontext model using a fixed probability of 0.5, by the coeff_sign_flagsyntax element.

The above coding technique for transform coefficient levels achievesgood coding performance. However, it contains some complexity due to thehigh adaptivity. This disallows to code multiple bins at the same time(except for the bins coded in the bypass mode). There are two reasonsfor that. First, the context model representing a probability modelitself is adaptive and is updated after the coding of each bin. So, theprobability model is different for each bin, even if the same contextmodel is selected for successive bins. Second, the context modelselection is often depending on previous coded syntax elements which aredifferent for each scan position. Thus, the state-of-the-art conceptdoesn't allow the coding multiple bins to reduce the complexity in theentropy coding stage and also doesn't exploit the advantages of the PIPEentropy coder, where the coding of multiple bins allows low complexityimplementation.

In accordance with an embodiment, a modified coding technique fortransform coefficient levels is described which is configurable todifferent levels of adaption and thus allows to make use of the multibin decoding ability of PIPE in low-complexity mode while be able tobenefit from high adaption in high-efficiency mode. The scalability isachieved by having a unified scan order for both complexity levels butdifferent modes for context derivations. The syntax elements used forthe coding of the transform coefficient levels are exactly the same asdescribed in above. But compared to that, the difference lies in thescan order for the remaining absolute transform levels and how subsetsare formed. Furthermore, when PIPE is employed as entropy coder, thesign information represents by coeff_sign_flag of a transform block cancoded directly, e.g. can write and read directly from the bitstream inone pass.

Instead of using 4×4 subsets and scan them separately by zigzag scan,while the subsets are scanned in reverse zigzag scan order, theembodiment described now uses the same scan order as for thesignificance map. The subsets are created along a specific number ofscan positions. According to this, first, the significance map is codedwith a specific scan order, where the scan direction can be reverse. Asdescribed in the above technique, the scan order can be differentdepending on the prediction mode and the transformation block size. Forthe remaining absolute transform coefficient levels, the same scan orderas for the significance map is used, while the scan order can bereversed compare to the scan of the significance map.

As mentioned before, the subsets are created along successive scanpositions instead of partitioning the transform block into 4×4sub-blocks. The creation of subsets along a specific number ofsuccessive scan positions is used for all configurations in the presentembodiment.

As previously mentioned, the complexity is configurable by the degree ofadaption. Following, three configurations are described. The firstconfiguration is suitable to achieve high coding performance, the secondfor low complexity processing, and the third is a trade-offconfiguration between complexity and coding performance. The threeconfiguration examples demonstrate how the context adaption can beconfigured, while the entropy coding engine in from of the decoders 322,for example, is the same for all configurations.

For the first configuration, the context modelling stage is the same asin the above technique. The only difference is the grouping intosubsets. Instead of grouping 4×4 spatial positions into a sub-block orsubsets, the embodiments present here groups several successive scanpositions into a subset as mentioned before. The context offset orcontext set derivation is not modified compared to the above outlinedtechnique and the context modelling parameters for the syntax elementsare also reset for each subset. In case of low complexity configuration,the same context model with fixed probability is used for the wholesubset. This concept allows the coding of multiple bins when PIPE isemployed as entropy coder. For that configuration, the adaptivity of thecontext model selection only depends on the scan position, but not onthe previously coded syntax elements, and the context models are notupdating their internal probability model representation. Instead ofusing the local template to derive the context model for each scanposition of the significance map, a specific amount number of successivescan positions are also grouped together into a subset and each subsetis coded using the same context model, while the probability for thatcontext model is fixed. Note that the subsets of the significance mapalso exist, but the local template is allowed to evaluate scan positionslying in other subsets. After specifying the significance map, theremaining absolute transform levels are coded. The same concept as forthe significance map is used for the remaining absolute levels. Hence, asuccessive number of significant scan positions are grouped together asa subset, and each subset is coded with the same context model for thecoeff_abs_greater1 syntax element. Next, the grouping mechanism is donefor the syntax elements coeff_abs_greater2. A successive number of scanpositions which are known as to be greater than one are grouped togetherto a set and the set is coded with a context model with fixedprobability. The remaining absolute transform coefficient level and thesign is coded as in the above outlined comparison technique, while theRice parameter is reset on the beginning of each subset along a specificnumber of scan positions and the sign information are coded in one step.The last configuration is a trade-off between complexity and codingperformance. In this configuration, the subset creation for each syntaxelement related to the coding of the transform coefficient levels havingthe same context model as it's done in the low complexity case. But incontrast to the low complexity configuration, the context model of asubset is updated after all bins related to that syntax element to thatsubset have been coded. Regardless of the chosen configuration, the PIPEconcept allows to code the sign information (represented bycoeff_sign_flag) of a transform block directly (e.g. can be written andread directly from the bitstream in one pass).

In the embodiment, the significance map is coded in the forward scanorder, while the remaining absolute transform coefficient levels arecoded in reverse scan order. Forward scan order may generally lead fromthe DC value to the highest frequency component as exemplarily shown inFIG. 6 such as along one-dimensional path 274, while the reverse ordertravels along the same path but in opposed direction. In anotherembodiments, the coding of the significance map and the remainingabsolute levels are coded in reverse scan order. In a furtherembodiment, all syntax elements related to the coding of absolutetransform levels are coded in a forward scan order.

In an embodiment, the size of a subset is fixed to 16 and is stretchedalong the scan positions of the transform block. In another embodiment,the size of the subset is fixed to 4 and is also stretch along the scanpositions of the transform block. In a further embodiment, the size ofthe subsets is variable depending on the spatial location of thetransform block. In that embodiment, the size of a subset is smaller forlow frequency area of the transform block, e.g. for the first scanpositions of the transform block, and becomes greater for higher scanpositions of the transform block. In a further embodiment, the size ofthe subsets equals to the width of the transform block. In anotherembodiment, each subset includes the scan positions along a diagonal ofthe transform block. In this embodiment, the diagonal is defined fromtop right to bottom left of the transform block.

In an embodiment, the subsets are created along a specific number ofsuccessive scan positions, for which the specific syntax element has tobe coded. In that case, the subsets are created as follows. The subsetsfor the coeff_significant_flag are stretched along the scan positions ofthe transform block, while the size of the subsets can be fixed orvariable. Next, for the coeff_abs_greater1 syntax element, depends onthe configuration how the size of the subsets are, and only scanpositions with coeff_significant_flag equal to 1 formed a subset. Forexample, in the fixed case, the scan positions 0-4 form a subset for thecoeff_significant_flag and the remaining absolute levels related syntaxelements. In contrast for this embodiment, if the subset size is 4 forthe coeff_significant_flag syntax element and the remaining absolutelevels related syntax elements, the scan positions 0-4 form the firstsubset and 4-8 form the second subset for coeff_significant_flag. Butonly scan positions with coeff_significant_flag equal to 1 form thesubset for coeff_abs_greater1. So, the subset for the coeff_abs_greater1has the same size as for coeff_significant_flag, but can stretch from0-6, if there are exact 4 coeff_significant_flag syntax element equal to1 in this range and the first at the scan position 0 and the fourth atthe scan position 6. This leads to a variable subset range. In anotherembodiment, the variable stretch length for a given subset size isapplied for the coeff_abs_greater2 syntax element. In a furtherembodiment, this concept is also used for the coeff_abs_minus3 syntaxelement.

In an embodiment, the context modelling for coeff_significant_flagemploys a local template and for the remaining absolute transformlevels, the context modelling is the same as in the state-of-the-art.This embodiment leads to the high efficiency configuration. In anotherembodiment, the context modelling for all syntax elements is dependingon the subsets only. In this embodiment, the context modelling isderived by the number of subsets. As an example, the transform blocksize is 8×8 and the subset size is fixed to 16 and stretch along thescan positions of transform block. In this case, the transform block has4 subsets and each subset uses a different context model. If the subsetsstretch along scan positions where the syntax element is coded, at least4 subsets exist for the coeff_significant_flag, while the number ofsubsets for the coeff_abs_greater1 syntax element depends on the numberof significant position. Assume that there are 24 significant scanpositions. So, the first 16 scan positions formed the first subset andthe remaining 8 scan positions formed the second subset. Again, for eachsubset, the specific context model is selected according to the subsetnumber.

In a further embodiment, the context modelling of a subset, e.g. thecontext model set or context model offset for coeff_abs_greater1 andcoeff_abs_greater2, depends on previously decoded subsets. In anotherembodiment, the context modelling of subset depends on the last ndecoded bins, where n is the number of bins that can be coded in asingle step. In a further embodiment, the context modelling stage of thesubset depends on the size of the subset. This embodiment is suitable incase of variable subset size.

For example, the reconstructor 404 may be configured to reconstruct atransform block 200 of transform coefficient levels 202 based on aportion of the sequence of syntax elements independent from thehigh-efficiency mode or the low-complexity mode being activated, theportion of the sequence 327 of syntax elements comprising, in anun-interleaved manner, significance map syntax elements defining asignificance map indicating positions of non-zero transform coefficientlevels within the transform block 200, and then (followed by) levelsyntax elements defining the non-zero transform coefficient levels. Inparticular, the following elements may be involved: end position syntaxelements (last_significant_pos_x, last_signficant_pos_y) indicating aposition of a last non-zero transform coefficient level within thetransform block; first syntax elements (coeff_significant_flag) togetherdefining a significance map and indicating, for each position along aone-dimensional path (274) leading from a DC position to the position ofthe last non-zero transform coefficient level within the transform block(200), as to whether the transform coefficient level at the respectiveposition is non-zero or not; second syntax elements (coeff_abs_greater1)indicating, for each position of the one-dimensional path (274) where,according to the first binary syntax elements, a non-zero transformcoefficient level is positioned, as to whether the transform coefficientlevel at the respective position is greater than one; and third syntaxelements (coeff_abs_greater2, coeff_abs_minus3) revealing, for eachposition of the one-dimensional path where, according to the firstbinary syntax elements, a transform coefficient level greater than oneis positioned, an amount by which the respective transform coefficientlevel at the respective position exceeds one.

The order among the end position syntax elements, the first, the secondand the third syntax elements may be same for the high-efficiency modeand the low-complexity mode, and the selector 402 may be configured toperform the selection among the entropy decoders 322 for symbols fromwhich the de-symbolizer 314 obtains the end position syntax elements,first syntax elements, second syntax elements and/or the third syntaxelements, differently depending on the low-complexity mode or thehigh-efficiency mode being activated.

In particular, the selector 402 may be configured, for symbols of apredetermined symbol type among a subsequence of symbols from which thede-symbolizer 314 obtains the first syntax elements and second syntaxelements, to select for each symbol of the predetermined symbol type oneof a plurality of contexts depending on previously retrieved symbols ofthe predetermined symbol type among the subsequence of symbols andperform the selection depending on a probability model associated withthe selected context in case of the high-efficiency mode beingactivated, and perform the selection in a piece wise constant mannersuch that the selection is constant over consecutive continuous subpartsof the subsequence in case of the low-complexity mode be activated. Asdescribed above, the subparts may be measured in the number of positionsover which the respective subpart extends when measured along theone-dimensional path 274, or in the number of syntax elements of therespective type already coded with the current context. That is, thebinary syntax elements coeff_signficant_flag, coeff_abs_greater1 andcoeff_abs_greater2, for example, are coded context adaptively withselecting the decoder 322 based on the probability model of the selectedcontext in HE mode. Probability adaptation is used as well. In LC mode,there are also different contexts which are used for each of the binarysyntax elements coeff_signficant_flag, coeff_abs_greater1 andcoeff_abs_greater2. However, for each of these syntax elements, thecontext is kept static for the first portion along path 274 withchanging the context merely at a transition to the next, immediatelyfollowing portion along the path 274. For example, each portion maydefined to be 4, 8, 16 positions of block 200 long, independent from asto whether for the respective position the respective syntax element ispresent or not. For example, coeff_abs_greater1 and coeff_abs_greater2are merely present for significant positions, i.e. positions where—orfor which —coeff_significant_flag is 1. Alternatively, each portion maydefined to be 4, 8, 16 syntax elements long, independent from as towhether for the thus resulting respective portion extends over a highernumber of block positions. For example, coeff_abs_greater1 andcoeff_abs_greater2 are merely present for significant positions, andthus, portions of four syntax elements each may extend over more than 4block positions due to positions therebetween along path 274 for whichno such syntax element is transmitted such as no coeff_abs_greater1 andcoeff_abs_greater2 because the respective level at this position iszero.

The selector 402 may be configured to, for the symbols of thepredetermined symbol type among the subsequence of symbols from whichthe de-symbolizer obtains the first syntax elements and second syntaxelements, select for each symbol of the predetermined symbol type theone of a plurality of contexts depending on a number of previouslyretrieved symbols of the predetermined symbol type within thesubsequence of symbols, which have a predetermined symbol value andbelong to the same subpart, or a number of previously retrieved symbolsof the predetermined symbol type within the sequence of symbols, whichbelong to the same subpart. The first alternative has been true forcoeff_abs_greater1 and the secondary alternative has be true forcoeff_abs_greater2 in accordance with the above specific embodiments.

Further, the third syntax elements revealing, for each position of theone-dimensional path where, according to the first binary syntaxelements, a transform coefficient level greater than one is positioned,an amount by which the respective transform coefficient level at therespective position exceeds one, may comprise integer-valued syntaxelements, i.e. coeff_abs_minus3, and the desymbolizer 314 may beconfigured to use a mapping function controllable by a control parameterto map a domain of symbol sequence words to a co-domain of theinteger-valued syntax elements, and to set the control parameter perinteger-valued syntax element depending on integer-valued syntaxelements of previous third syntax elements if the high-efficiency modeis activated, and perform the setting in a piecewise constant mannersuch that the setting is constant over consecutive continuous subpartsof the subsequence in case of the low-complexity mode being activated,wherein the selector 402 may configured to select a predetermined one ofthe entropy decoders (322) for the symbols of symbol sequence wordsmapped onto the integer-valued syntax elements, which is associated withan equal probability distribution, in both the high-efficiency mode andthe low-complexity mode. That is, even the desymbolizer may operatedependent on the mode selected be switch 400 is illustrated by dottedline 407. Instead of a piecewise constant setting of the controlparameter, the desymbolizer 314 may keep the control parameter constantduring the current slice, for example, or constant globally in time.

Next, a complexity-scalable context modelling is described.

The evaluation of the same syntax element of the top and the leftneighbour for the derivation of the context model index is a commonapproach and is often used in the HE case, e.g. for the motion vectordifference syntax element. However, this evaluation necessitates morebuffer storage and disallows the direct coding of the syntax element.Also, to achieve higher coding performance, more available neighbourscan be evaluated.

In an embodiment, all context modelling stage evaluating syntax elementsof neighbor square or rectangle blocks or prediction units are fixed toone context model. This is equal to the disabling of the adaptivity inthe context model selection stage. For that embodiment, the contextmodel selection depending on the bin index of the bin string after abinarization is not modified compared to the current design for CABAC.In another embodiment, additional to the fixed context model for syntaxelements employ the evaluation of neighbors, also the context model forthe different bin index are fixed. Note that the description does notinclude the binarization and context model selection for the motionvector difference and the syntax elements related to the coding of thetransform coefficient levels.

In an embodiment, only the evaluation of the left neighbor is allowed.Ibis leads to reduced buffer in the processing chain because the lastblock or coding unit line has not to be stored anymore. In a furtherembodiment, only neighbors lying in the same coding unit are evaluated.

In an embodiment, all available neighbors are evaluated. For example, inaddition to the top and the left neighbor, the top left, the top right,and the bottom left neighbor are evaluated in case of availability.

That is, the selector 402 of FIG. 18 may be configured to use, for apredetermined symbol relating to a predetermined block of the mediadata, previously retrieved symbols of the sequence of symbols relatingto a higher number of different neighboring blocks of the media data incase of the high-efficiency mode being activated in order to select oneof a plurality of contexts and perform the selection among the entropydecoders 322 depending on a probability model associated with theselected context. That is, the neighboring blocks may neighbor in timesand/or spatial domain. Spatially neighboring blocks are visible, forexample, in FIGS. 1 to 3 . Then, selector 402 may be responsive to themode selection by mode switch 400 to perform a contact adaptation basedon previously retrieved symbols or syntax elements relating to a highernumber of neighboring blocks in case of the HE mode compared to the LCmode thereby reducing the storage overhead as just-described.

Next, a reduced-complexity coding of motion vector differences inaccordance with an embodiment is described.

In the H.264/AVC video codec standard, a motion vector associated with amacroblock is transmitted by signaling the difference (motion vectordifference—mvd) between the motion vector of the current macroblock andthe median motion vector predictor. When the CABAC is used as entropycoder, the mvd is coded as follows. The integer-valued mvd is split intoan absolute and the sign part. The absolute part is binarized using acombination of truncated unary and 3rd order Exp-Golomb, referred to asthe prefix and the suffix of the resulting bin string. The bins relatedto the truncated unary binarization is coded using context models, whilebins related to the Exp-Golomb binarization is coded in a bypass mode,i.e. with a fixed probability of 0.5 with CABAC. The unary binarizationworks as follows. Let the absolute integer-value of the mvd be n, thenthe resulting bin string consists of n times ‘1’ and one trailing ‘0’.As an example, let n=4, then the bin string is ‘11110’. In case oftruncated unary, a limit exists and if the value excesses this limit,the bin string consists of n+1 times ‘1’. For the case of mvd, the limitis equal to 9. That means if an absolute mvd is equal to or greater than9 is coded, resulting in 9 times ‘1’, the bin string consists of aprefix and a suffix with Exp-Golomb binarization. The context modellingfor the truncated unary part is done as follows. For the first bin ofthe bin string, the absolute mvd values from the top and the leftneighbour macroblocks are taken if available (if not available, thevalue is inferred to be 0). If the sum for the specific component(horizontal or vertical direction) is greater than 2, the second contextmodel is selected, if the absolute sum is greater than 32, the thirdcontext model is selected, otherwise (the absolute sum is smaller than3) the first context model is selected. Furthermore, the context modelsare different for each component. For the second bin of the bin string,the fourth context model is used and the fifth context model is employedfor the remaining bins of the unary part. When the absolute mvd is equalto or greater than 9, e.g. all bins of the truncated unary part areequal to ‘1’, the difference between the absolute mvd value and 9 iscoded in a bypass mode with 3rd order Exp-Golomb binarization. In thelast step, the sign of the mvd is coded in a bypass mode.

The latest coding technique for the mvd when using CABAC as entropycoder is specified in the current Test Model (HM) of the High EfficiencyVideo Coding (HEVC) project. In HEVC, the block sizes are variable andthe shape specified by a motion vector is referred to as prediction unit(PU). The PU size of the top and the left neighbor may have other shapesand sizes than the current PU. Therefore, whenever relevant, thedefinition of top and the left neighbor are referred now as top and leftneighbor of the top-left corner of the current PU. For the codingitself, only the derivation process for the first bin may be changed inaccordance with an embodiment. Instead of evaluating the absolute sum ofthe MV from the neighbors, each neighbor may be evaluated separately. Ifthe absolute MV of a neighbor is available and greater than 16, thecontext model index may be incremented resulting in the same number ofcontext models for the first bin, while the coding of the remainingabsolute MVD level and the sign is exactly the same as in H.264/AVC.

In the above outlined technique on coding of the mvd, up to 9 bins haveto be coded with a context model, while the remaining value of an mvdcan be coded in a low complexity bypass mode together with the signinformation. This present embodiment describes a technique to reduce thenumber of bins coded with context models resulting in increased numberof bypass and reduces the number of context models necessitated for thecoding of mvd. For that, the cut-off value is decreased from 9 to 1 or2. That means only the first bin specifying if the absolute mvd isgreater than zero is coded using context model or the first and thesecond bin specifying if the absolute mvd is greater than zero and oneis coded using context model, while the remaining value is coded in thebypass mode and/or using a VLC code. All bins resulting from thebinarization using the VLC code—not using the unary or truncated unarycode—are coded using a low complexity bypass mode. In case of PIPE, adirect insertion into and from the bitstream are possible. Moreover, adifferent definition of the top and the left neighbor to derive bettercontext model selection for the first bin, may be used, if ever.

In an embodiment, Exp-Golomb codes are used to binarize the remainingpart of the absolute MVD components. For that, the order of theExp-Golomb code is variable. The order of the Exp-Golomb code is derivedas follows. After the context model for the first bin, and therefore theindex of that context model, is derived and coded, the index is used asthe order for the Exp-Golomb binarization part. In this embodiment, thecontext model for the first bin is ranged from 1-3 resulting in theindex 0-2, which are used as the order of the Exp-Golomb code. Thisembodiment can be used for the HE case.

In an alternative to the above outlined technique of using two timesfive contexts in coding of the absolute MVD, in order to code the 9unary code binarization bins, 14 context models (7 for each component)could be used as well. For example, while the first and second bins ofthe unary part could be could be coded with four different contexts asdescribed before, a fifth context could be used for the third bin and asixth context could be used with respect to the forth bin, while thefifth to ninth bins are coded using a seventh context. Thus, in thiscase even 14 contexts would be necessitated, and merely the remainingvalue can be coded in a low complexity bypass mode. A technique toreduce the number of bins coded with context models resulting inincreased number of bypass and reduce the number of context modelsnecessitated for the coding of MVD, is to decrease the cut-off valuesuch as, for example, from 9 to 1 or 2. That means only the first binspecifying if the absolute MVD is greater than zero would be coded usinga context model or the first and the second bin specifying if theabsolute MVD is greater than zero and one would be coded using arespective context model, while the remaining value is coded with a VLCcode. All bins resulting from the binarization using the VLC code arecoded using a low complexity bypass mode. In case of PIPE, a directinsertion into and from the bitstream is possible. Furthermore, thepresented embodiment uses another definition of the top and the leftneighbor to derive better context model selection for the first bin. Inaddition to this, the context modeling is modified in a way so that thenumber of context models necessitated for the first or the first andsecond bin is decreased leading to a further memory reduction. Also, theevaluation of the neighbours such as the above neighbour can be disabledresulting in the saving of the line buffer/memory necessitated forstorage of mvd values of the neighbours. Finally, the coding order ofthe components may be split in a way allowing the coding of the prefixbins for both components (i.e. bins coded with context models) followedby the coding of bypass bins.

In an embodiment, Exp-Golomb codes are used to binarize the remainingpart of the absolute mvd components. For that, the order of theExp-Golomb code is variable. The order of the Exp-Golomb code may bederived as follows. After the context model for the first bin, andtherefore the index of that context model is derived, the index is usedas the order for the Exp-Golomb binarization. In this embodiment, thecontext model for the first bin is ranged from 1-3 resulting in theindex 0-2, which is used as the order of the Exp-Golomb code. Thisembodiment can be used for the HE case and the number of context modelsis reduced to 6. In order to reduce the number of context models againand therefore to save memory, the horizontal and the vertical componentsmay share the same context models in a further embodiment. In that case,only 3 context models are necessitated. Furthermore, only the leftneighbour may be taken into account for the evaluation in a furtherembodiment of the invention. In this embodiment, the threshold can beunmodified (e.g. only single threshold of 16 resulting in Exp-Golombparameter of 0 or 1 or single threshold of 32 resulting in Exp-Golombparameter of 0 or 2). This embodiment saves the line buffer necessitatedfor the storage of mvd. In another embodiment, the threshold is modifiedand is equal to 2 and 16. For that embodiment, in total 3 context modelsare necessitated for the coding of the mvd and the possible Exp-Golombparameter is ranged from 0-2. In a further embodiment, the threshold isequal to 16 and 32. Again, the described embodiment is suitable for theHE case.

In a further embodiment of the invention, the cut-off value is decreasedfrom 9 to 2. In this embodiment, the first bin and the second bin may becoded using context models. The context model selection for the firstbin can be done as in the state-of-the-art or modified in a waydescribed in the embodiment above. For the second bin, a separatecontext model is selected as in the state-of-the-art. In a furtherembodiment, the context model for the second bin is selected byevaluating the mvd of the left neighbour. For that case, the contextmodel index is the same as for the first bin, while the availablecontext models are different than those for the first bin. In total, 6context models are necessitated (note that the components sharing thecontext models). Again, the Exp-Golomb parameter may depend on theselected context model index of the first bin. In another embodiment ofthe invention, the Exp-Golomb parameter is depending on the contextmodel index of the second bin. The described embodiments of theinvention can be used for the HE case.

In a further embodiment of the invention, the context models for bothbins are fixed and not derived by evaluating either the left or theabove neighbours. For this embodiment, the total number of contextmodels is equal to 2. In a further embodiment of the invention, thefirst bin and the second bin shares the same context model. As a result,only one context model is necessitated for the coding of the mvd. Inboth embodiments of the invention, the Exp-Golomb parameter may be fixedand be equal to 1. The described embodiment of the invention is suitablefor both HE and LC configuration.

In another embodiment, the order of the Exp-Golomb part is derivedindependently from the context model index of the first bin. In thiscase, the absolute sum of the ordinary context model selection ofH.264/AVC is used to derive the order for the Exp-Golomb part. Thisembodiment can be used for the HE case.

In a further embodiment, the order of the Exp-Golomb codes is fixed andis set to 0. In another embodiment, the order of the Exp-Golomb codes isfixed and set to 1. In an embodiment, the order of the Exp-Golomb codesis fixed to 2. In a further embodiment, the order of the Exp-Golombcodes is fixed to 3. In a further embodiment, the order of theExp-Golomb codes is fixed according the shape and the size of thecurrent PU. The presented embodiments can be used for the LC case. Notethat the fixed order of the Exp-Golomb part are considered with reducednumber of bins coded with context models.

In an embodiment, the neighbors are defined as follows. For the abovePU, all PUs covers the current PU are taken into account and the PU withthe largest MV used. This is done also for the left neighbor. All PUscovers the current PU are evaluated and the PU with the largest MV isused. In another embodiment, the average absolute motion vector valuefrom all PUs cover the top and the left border the current PU is used toderive the first bin.

For the presented embodiments above, it is possible to change the codingorder as follows. The mvd have to be specified for the horizontal andvertical direction one after another (or vice versa). Thus, two binstrings have to be coded. In order to minimize the number of modeswitching for the entropy coding engine (i.e. the switch between thebypass and the regular mode), it is possible to code the bins coded withcontext models for both components in the first step followed by thebins coded in bypass mode in the second step. Note that this is areordering only.

Please note that the bins resulting from the unary or truncated unarybinarization can also be represented by an equivalent fixed lengthbinarization of one flag per bin index specifying whether the value isgreater than the current bin index. As an example, the cut-off value fortruncated unary binarization of mvd is set to 2 resulting in codewords0, 10, 11 for values 0, 1, 2. In the corresponding fixed lengthbinarization with one flag per bin index, one flag for bin index 0 (i.e.the first bin) specifies whether the absolute mvd value is greater than0 or not and one flag for the second bin with bin index 1 specifieswhether the absolute mvd value is greater than 1 or not. When the secondflag is only coded when the first flag is equal to 1, this results inthe same codewords 0, 10, 11.

Next, complexity-scalable representation of the internal state ofprobability models in accordance with an embodiment as described.

In the HE-PIPE setup, the internal state of a probability model isupdated after encoding a bin with it. The updated state is derived by astate transition table lookup using the old state and the value of thecoded bin. In the case of CABAC, a probability model can take 63different states where each state corresponds to a model probability inthe interval (0.0, 0.5). Each of these states is used to realize twomodel probabilities. In addition to the probability assigned to thestate, 1.0 minus the probability is also used and a flag called valMpsstores the information whether the probability or 1.0 minus theprobability is used. This leads to a total of 126 states. To use such aprobability model with the PIPE coding concept, each of the 126 statesneeds to be mapped to one of the available PIPE coders. In currentimplementations of PIPE coders, this is done by using a lookup-table. Anexample of such a mapping is depicted in Table A.

In the following, an embodiment is described how the internal state of aprobability model can be represented to avoid using a lookup table toconvert the internal state to a PIPE index. Solely some simple bitmasking operations are needed to extract the PIPE index from theinternal state variable of the probability model. This novelcomplexity-scalable representation of the internal state of aprobability model is designed in a two level manner. For applicationswhere low complexity operation is mandatory only the first level isused. It describes only the pipe index and the flag valMps that is usedto encode or decode the associated bins. In the case of the describedPIPE entropy coding scheme, the first level can be used to differentiatebetween 8 different model probabilities. Thus, the first level wouldneed 3 bit for the pipeIdx and one further bit for the valMps flag. Withthe second level each of the coarse probability ranges of the firstlevel is refined into several smaller intervals that support thepresentation of probabilities at higher resolutions. This more detailedpresentation enables the more exact operation of probability estimators.In general, it is suitable for coding applications that aim towards highRD-performances. As an example this complexity-scaled representation ofthe internal state of probability models with the usage of PIPE isillustrated as follows:

First Level Second Level b₇ b₆ b₅ b₄ b₃ b₂ b₁ b₀ MPS PIPE Idx (0-7)Refinement Idx (0-15)

The first and the second level are stored in a single 8 bit memory. 4bits are necessitated to store the first level—an index that defines thePIPE index with the value of the MPS on the most significant bit- andanother 4 bits are used to store the second level. To implement thebehaviour of the CABAC probability estimator, each PIPE index has aparticular number of allowed refinement indices depending on how manyCABAC states were mapped on the PIPE index. E.g. for the mapping inTable A, the number of CABAC states per PIPE index is depicted in TableB.

TABLE B Number of CABAC states per PIPE index for the example of TableA. PIPE idx 0 1 2 3 4 5 6 7 Number of 3 7 5 7 10 14 16 1 CABAC states

During the encoding or decoding process of a bin the PIPE index andvalMps can be accessed directly by employing simple bit mask or bitshift operations. Low complexity coding processes necessitate the 4 bitsof the first level only and high efficiency coding processes canadditionally utilize the 4 bits of the second level to perform theprobability model update of the CABAC probability estimator. Forcarrying out this update, a state transition lookup-table can bedesigned that does the same state transitions as the original table, butusing the complexity-scalable two-level representation of states. Theoriginal state transition table consists of two times 63 elements. Foreach input state, it contains two output states. When using thecomplexity-scalable representation, the size of the state transitiontable does not exceed two times 128 elements which is an acceptableincrease of table size. This increase depends on how many bits are usedto represent the refinement index and to exactly emulate the behavior ofthe CABAC probability estimator, four bits are needed. However, adifferent probability estimator could be used, that can operate on areduced set of CABAC states such that for each pipe index no more than 8states are allowed. Therefore memory consumption can be matched to thegiven complexity level of the coding process by adapting the number ofbits used to represent the refinement index. Compared to the internalstate of model probabilities with CABAC—where 64 probability stateindices exist—the usage of table lookups to map model probabilities to aspecific PIPE code is avoided and no further conversion is required.

Next, a complexity-scalable context model updating in accordance with anembodiment is described.

For updating a context model, its probability state index may be updatedbased on one or more previously coded bins. In the HE-PIPE setup, thisupdate is done after encoding or decoding of each bin. Conversely, inthe LC-PIPE setup, this update may never be done.

However, it is possible to do an update of context models in acomplexity-scalable way. That is, the decision whether to update acontext model or not may be based on various aspects. E.g., a codersetup could do no updates for particular context models only like e.g.the context models of syntax element coeff_signficant_flag, and doupdates for all other context models.

In other words, the selector 402 could be configured to, for symbols ofeach of a number of predetermined symbol types, perform the selectionamong the entropy decoders 322 depending on a respective probabilitymodel associated the respective predetermined symbol such that thenumber of predetermined symbol types is lower in the low complexity modethan compared to the high-efficiency mode

Furthermore, criteria for controlling whether to update a context modelor not could be, e.g. the size of a bitstream packet, the number of binsdecoded so far, or the update is done only after coding a particularfixed or variable number of bins for a context model.

With this scheme for deciding whether to update context models or not,complexity-scalable context model updating can be implemented. It allowsfor increasing or decreasing the portion of bins in a bitstream forwhich context model updates are done. The higher the number of contextmodel updates, the better is the coding efficiency and the higher thecomputational complexity. Thus, complexity-scalable context modelupdating can be achieved with the described scheme.

In an embodiment, the context model update is done for bins of allsyntax elements except the syntax elements coeff_signficant_flag,coeff_abs_greater1, and coeff_abs_greater2.

In a further embodiment, the context model update is done for bins ofthe syntax elements coeff_signficant_flag, coeff_abs_greater1, andcoeff_abs_greater2 only.

In a further embodiment, the context model update is done for allcontext models when encoding or decoding of a slice starts. After aparticular predefined number of transform blocks being processed,context model update is disabled for all context models until the end ofthe slice is reached.

For example, the selector 402 may be configured to, for symbols of apredetermined symbol type, perform the selection among the entropydecoders 322 depending on a probability model associated with thepredetermined symbol type along with or without updating the associatedprobability model, such that a length of a learning phase of thesequence of symbols over which the selection for the symbols of thepredetermined symbol type is performed along with the update, is shorterin the low complexity mode than compared to the high-efficiency mode.

A further embodiment is identical to the previously describedembodiment, but it uses the complexity-scalable representation of theinternal state of context models in a way, such that one table storesthe “first part” (valMps and pipeIdx) of all context models and a secondtable stores the “second part” (refineIdx) of all context models. At thepoint, where the context model updating is disabled for all contextmodels (as described in the previous embodiment), the table storing the“second part” is not needed any longer and can be discarded.

Next, context model updating for a sequence of bins in accordance withan embodiment is described.

In the LC-PIPE configuration, the bins of syntax elements of typecoeff_significant_flag, coeff_abs_greater1, and coeff_abs_greater2 aregrouped into subsets. For each subset, a single context model is used toencode its bins. In this case, a context model update may be done aftercoding of a fixed number of bins of this sequence. This is denotedmulti-bin update in the following. However, this update may differ fromthe update using only the last coded bin and the internal state of thecontext model. E.g., for each bin that was coded, one context modelupdate step is conducted.

In the following, examples are given for the encoding of an exemplarysubset consisting of 8 bins. The letter ‘b’ denotes the decoding of abin and the letter ‘u’ denotes the update of the context model. In theLC-PIPE case only the bin decoding is done without doing context modelupdates:

-   -   b b b b b b b b

In the HE-PIPE case, after decoding of each bin, a context model updateis done:

-   -   b u b u b u b u b u b u b u b u

In order to somewhat decrease the complexity, the context model updatemay be done after a sequence of bins (in this example after each 4 bins,the updates of these 4 bins are done):

-   -   b b b b u u u u b b b b u u u u

That is, the selector 402 may be configured to, for symbols of apredetermined symbol type, perform the selection among the entropydecoders 322 depending on a probability model associated with thepredetermined symbol type along with or without updating the associatedprobability model such that a frequency at which the selection for thesymbols of the predetermined symbol type is performed along with theupdate, is lower in the low complexity mode than compared to thehigh-efficiency mode

In this case, after the decoding of 4 bins, 4 update steps follow basedon the 4 bins just-decoded. Note that these four update steps can beconducted in one single step by using a lookup special lookup-table.This lookup table stores for each possible combination of 4 bins andeach possible internal state of the context model the resulting newstate after the four conventional update steps.

In a certain mode, the multi-bin update is used for syntax elementcoeff_significant_flag. For bins of all other syntax elements, nocontext model update is used. The number of bins that are coded before amulti-bin update step is done is set to n. When the number of bins ofthe set is not divisible by n, 1 to n−1 bins remain at the end of thesubset after the last multi-bin update. For each of these bins, aconventional single-bin update is done after coding all of these bins.The number n may be any positive number greater than 1. Another modecould be identical to the previous mode, except that multi-bin update isdone for arbitrary combinations of coeff_signficant_flag,coeff_abs_greater1 and coeff_abs_greater2 (instead ofcoeff_signficant_flag only). Thus, this mode would be more complex thanthe other. All other syntax elements (where multi-bin update is notused) could be divided into two disjoint subsets where for one of thesubsets, single bin update is used and for the other subset no contextmodel update is used. Any possible disjoint subsets are valid (includingthe empty subset).

In an alternative embodiment, the multi-bin update could be based on thelast m bins only that are coded immediately before the multi-bin updatestep. m may be any natural number smaller than n. Thus, decoding couldbe done like:

-   -   b b b b u u b b b b u u b b b b u u b b b b . . .    -   with n=4 and m=2.

That is, the selector 402 may be configured to, for symbols of apredetermined symbol type, perform the selection among the entropydecoders 322 depending on a probability model associated with thepredetermined symbol type, along with updating the associatedprobability model every n-th symbol of the predetermined type based on mmost recent symbols of the predetermined symbol type such that the ration/m is higher in the low complexity mode than compared to thehigh-efficiency mode.

In a further embodiment, for syntax element coeff_signficant_flag, thecontext modeling scheme using a local template as described above forthe HE-PIPE configuration may be used to assign context models to binsof the syntax element. However, for these bins, no context model updateis used.

Further, the selector 402 may be configured to, for symbols of apredetermined symbol type, select one of a number of contexts dependingon a number of previously retrieved symbols of the sequence of symbolsand perform the selection among the entropy decoders 322 depending on aprobability model associated with the selected context, such that thenumber of contexts, and/or the number of previously retrieved symbols,is lower in the low complexity mode than compared to the high-efficiencymode.

Probability Model Initialization Using 8 Bit Initialization Values

This section describes the initialization process of thecomplexity-scalable internal state of probability models using aso-called 8 bit initialization value instead of two 8 bit values as isthe case in the state-of-the-art video coding standard H.264/AVC. Itconsists of two parts which are comparable to the initialization valuepairs used for probability models in CABAC of H.264/AVC. The two partsrepresent the two parameters of a linear equation to compute the initialstate of a probability model, representing a particular probability(e.g. in form of a PIPE index) from a QP:

-   -   The first part describes the slope and it exploits the        dependency of the internal state in respect to the quantization        parameter (QP) that is used during encoding or decoding.    -   The second part defines a PIPE index at a given QP as well as        the valMps.

Two different modes are available to initialize a probability modelusing the given initialization value. The first mode is denotedQP-independent initialization. It only uses the PIPE index and valMpsdefined in the second part of the initialization value for all QPs. Thisis identical to the case where the slope equals 0. The second mode isdenoted QP-dependent initialization and it additionally uses the slopeof the first part of the initialization value to alter the PIPE indexand to define the refinement index. The two parts of an 8 bitinitialization value is illustrated as follows:

First Part Second Part b₇ b₆ b₅ b₄ b₃ b₂ b₁ b₀ Slope Index PIPEProbability Index

It consists of two 4 bit parts. The first part contains an index thatpoints to 1 out of 16 different predefined slopes that are stored in anarray. The predefined slopes consist of 7 negative slopes (slope index0-6), one slope that equals zero (slope index 7) and 8 positive slopes(slope index 8-15). The slopes are depicted in Table C.

TABLE C Slope Index 0 1 2 3 4 5 6 7 Slope −239 −143 −85 −51 −31 −19 −110 Value Slope Index 8 9 10 11 12 13 14 15 Slope 11 19 31 51 85 143 239399 Value

All values are scaled by a factor of 256 to avoid the usage of floatingpoint operations. The second part is the PIPE index which embodies theascending probability of valMps=1 between the probability interval p=0and p=1. In other words, PIPE coder n must operate at a higher modelprobability than PIPE coder n−1. For every probability model one PIPEprobability index is available and it identifies the PIPE coder whoseprobability interval contains the probability of p_(valMPs=1) for QP=26.

TABLE D Mapping of the second part of the initialization value to PIPEcoders and valMps: UR = unary-to-rice-code, TB = three-bin- code, BP =bin-pipe-code, EP = equal probability (uncoded). PIPE Probability Index0 1 2 3 4 5 6 7 PIPE Coder UR5 UR4 UR3 UR2 TB BP2 BP3 EP MPS 0 0 0 0 0 00 0 PIPE Probability Index 8 9 10 11 12 13 14 15 PIPE Coder EP BP3 BP2TB UR2 UR3 UR4 UR5 MPS 1 1 1 1 1 1 1 1

The QP and the 8 bit initialization value are necessitated to calculatethe initialization of the internal state of the probability models bycomputing a simple linear equation in the form of y=m*(QP−QPref)+256*b.Note m defines the slope that is taken from Table C by using the slopeindex (the first part of the 8 bit initialization value) and b denotesthe PIPE coder at QPref=26 (the second part of the 8 bit initializationvalue: “PIPE Probability Index”). Then, valMPS is 1 and the pipeIdxequals (y−2048)>>8 if y is greater than 2047. Otherwise, valMPS is 0 andpipeIdx equals (2047−y)>>8. The refinement index equals (((y−2048) &255)*numStates)>>8 if valMPS equals 1. Otherwise, the refinement indexequals (((2047−y) & 255)*numStates)>>8. In both cases, numStates equalsthe number of CABAC states of the pipeIdx as depicted in Table B.

The above scheme can not only be used in combination with PIPE coders,but also in connection with the above-mentioned CABAC schemes. In theabsence of PIPE, the number of CABAC states, i.e. the probability statesbetween which the state transition in the probability update isperformed (pState_current[bin]), per PIPE Idx (i.e. the respective mostsignificant bits of pState_current[bin]) is then only a set ofparameters which realizes, in fact, a piece-wise linear interpolation ofthe CABAC state depending on the QP. Furthermore, this piece-wise linearinterpolation can also virtually be disabled in the case where theparameter numStates uses the same value for all PIPE Idx. For example,setting numStates to 8 for all cases yields a total of 16*8 states andthe computation of the refinement index simplifies to ((y−2048) &255)>>5 for valMPS equal 1 or ((2047−y)&255)>>5 for valMPS equal 0. Forthis case, mapping the representation using valMPS, PIPE idx, andrefinement idx back to the representation used by the original CABAC ofH.264/AVC is very simple. The CABAC state is given as (PIPEIdx<<3)+refinement Idx. This aspect is described further below withregard to FIG. 17 .

Unless the slope of the 8 bit initialization value equals zero or unlessthe QP equals 26 it is necessitated to compute the internal state byemploying the linear equation with the QP of the encoding or decodingprocess. In the case of the slope equaling to zero or that the QP of thecurrent coding process equals 26 the second part of 8 bit initializationvalue can be used directly for initializing the internal state of aprobability model. Otherwise the decimal part of the resulting internalstate can be further exploited to determine a refinement index in highefficiency coding applications by linear interpolation between thelimits of the specific PIPE coder. In this embodiment the linearinterpolation is executed by simply multiplying the decimal part withthe total number of refinement indices available for the current PIPEcoder and mapping the result to the closest integer refinement index.

The process of initialization of the internal state of the probabilitymodels could be varied with regard to the number of PIPE probabilityindex states. In particular, the double occurrence of the equal probablemode using PIPE coder E1, i.e. the use of two different PIPE indices todistinguish between MPS being 1 or 0, could be avoided as follows.Again, the process could be invoked during the start of parsing of theslice data, and the input of this process could an 8 bit initializationvalue as depicted in Table E, which would be, for example, transmittedwithin the bit stream for every context model to be initialized.

TABLE E Setup of the 8 bits of initValue for a probability model First 4bits Last 4 bits initValue bits b₇ b₆ b₅ b₄ b₃ b₂ b₁ b₀ VariableslopeIdx propIdx

The first 4 bits define a slope index and are retrieved by masking thebits b4-b7. For every slope index a slope (m) is specified and displayedin Table F.

TABLE F Values of variable m for slopeIdx slopeIdx 0 1 2 3 4 5 6 7 8 910 11 12 13 14 15 m −239 −143 −85 −51 −31 −19 −11 0 11 19 31 51 85 143239 399

Bits b0-b3, the last 4 bits of the 8 bit initialization value, identifythe probIdx and describe the probability at a predefined QP. probIdx 0indicates the highest probability for symbols with value 0 andrespectively, probIdx 14 indicates the highest probability for symbolswith value 1. Table G shows for each probIdx the corresponding pipeCoderand its valMps.

TABLE G Mapping of the last 4 bits part of the initialization value toPIPE coders and valMps: UR = unary-to-rice-code, TB = three-bin-code, BP= bin-pipe-code, EP = equal probability (uncoded) probIdx 0 1 2 3 4 5 67 8 9 10 11 12 13 14 pipeCoder UR5 UR4 UR3 UR2 TBC BP2 BP3 EP BP3 BP2TBC UR2 UR3 UR4 UR5 valMps 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1

With both values the calculation of the internal state could be done byusing a linear equation like y=m*x+256*b, where m denotes the slope, xdenotes the QP of the current slice and b is derived from the probIdx asshown in the following description. All values in this process arescaled by a factor of 256 to avoid the usage of floating pointoperations. The output (y) of this process represents the internal stateof the probability model at the current QP and is stored in a 8 bitmemory. As shown in G the internal state consists of the valMPs, thepipeIdx and the refineIdx.

TABLE H Setup of the internal state of a probability model First 4 bitsLast 4 bits initValue bits b₇ b₆ b₅ b₄ b₃ b₂ b₁ b₀ Variable valMpspipeIdx refineIdx

The assignment of the refineIdx and pipeIdx is similar to the internalstate of the CABAC probability models (pStateCtx) and is presented in H.

TABLE I Assignement of pipeldx, refineIdx and pStateCtx pipeIdx 0 1 2refineIdx 0 1 2 0 1 2 3 4 5 6 0 1 2 3 4 pStateCtx 0 1 2 3 4 5 6 7 8 9 1011 12 13 14 pipeIdx 3 4 refineIdx 0 1 2 3 4 5 6 0 1 2 3 4 5 6 7 8 9pStateCtx 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 pipeIdx 5refineIdx 0 1 2 3 4 5 6 7 8 9 10 11 12 13 pStateCtx 32 33 34 35 36 37 3839 40 41 42 43 44 45 pipeIdx 6 7 refineIdx 0 1 2 3 4 5 6 7 8 9 10 11 1213 14 15 0 pStateCtx 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62

In an embodiment the probIdx is defined at QP26. Based on the 8 bitinitialization value the internal state (valMps, pipeIdx and refineIdx)of a probability model is processed as described in the followingpseudo-code:

n = ( probIdx << 8 ) − m * 26 fullCtxState = max( 0, min( 3839, ( m *max( 0,min( 51, SliceQP_(Y) ) ) ) ) + n + 128 ) remCtxState =fullCtxState & 255 preCtxState = fullCtxState >> 8 if( preCtxState < 8 ){  pipeIdx = 7 − preCtxState  valMPS = 0 } else {  pipeIdx = preCtxState− 8  valMPS = 1 } offset = { 3, 7, 5, 7, 10, 14, 16, 1 } if( pipeIdx = =0 ) {  if( remCtxState <= 127)   remCtxState = 127 − remCtxState  else  remCtxState = remCtxState − 128  refineIdx = ( ( remCtxState << 1 ) * offset ) >> 8 } else {  if( valMPS = = 0 )   remCtxState = 255 −remCtxState  refineIdx = ( remCtxState *  offset[pipeldx] ) >> 8 }

As shown in the pseudo code the refineIdx is calculated by linearlyinterpolating between the interval of the pipeIdx and quantizing theresult to the corresponding refineIdx. The offset specifies the totalnumber of refineIdx for each pipeIdx. The interval [7, 8) offullCtxState/256 is divided in half. The interval [7, 7.5) is mapped topipeIdx=0 and valMps=0 and the interval [7.5, 8) is mapped to pipeIdx=0and valMps=1. FIG. 22 depicts the process of deriving the internal stateand displays the mapping of fullCtxState/256 to pStateCtx.

Note the slope indicates the dependency of the probIdx and the QP. Ifthe slopeIdx of the 8 bit initialization value equals 7 the resultinginternal state of the probability model is the same for all sliceQPs—hence the initialization process of the internal state isindependent from the current QP of the slice.

That is, selector 402 may initialize the pipe indices to be used indecoding the following portion of the datastream such as the wholestream or the next slice, using the syntax element indicating thequantization step size QP used in order to quantize the data of thisportion, such as the transform coefficient levels contained thereinusing this syntax element as an index into a table which may be commonfor both modes, LC and HE. The table such as table D may comprise pipeindices for each symbol type, for a respective reference QPref, or otherdata for each symbol type. Depending on the actual QP of the currentportion, the selector may compute a pipe index value using therespective table entry a indexed by the actual QP and QP itself, such asby multiplication a with (QP-QPref). The only difference in LC and HEmode: The selector computes the result merely at a lower accuracy incase of LC compared to HE mode. The selector may, for example, merelyuse the integer part of the computation result. In HE mode, the higheraccuracy remainder, such as the fractional part, is used to select oneof available refinement indices for the respective pipe index asindicated by the lower accuracy or integer part. The refinement index isused in HE mode (in potentially more seldomly also in LC mode) in orderto perform the probability adaptation such as by using theabove-mentioned table walk. When leaving the available indices for thecurrent pipe index at the higher bound, then the higher pipe index isselected next with minimizing the refinement index. When leaving theavailable indices for the current pipe index at the lower bound, thenthe next lower pipe index is selected next with maximizing therefinement index to the maximum available for the new pipe index. Thepipe index along with the refinement index define the probability state,but for the selection among the partial streams, the selector merelyuses the pipe index. The refinement index merely serves for tracking theprobability more closely, or in a finer accuracy.

The above discussion also showed, however, that a complexity scalabilitymay be achieved independent from the PIPE or CABAC coding concept ofFIG. 7-17 , using a decoder as shown in FIG. 19 . The Decoder of FIG. 19is for decoding a data stream 601 into which media data is coded, andcomprises a mode switch 600 configured to activate a low-complexity modeor a high efficiency mode depending on the data stream 601, as well as adesymbolizer 602 configured to desymbolize a sequence 603 of symbolsobtained—either directly or by entropy decoding, for example—from thedata stream 601 to obtain integer-valued syntax elements 604 using amapping function controllable by a control parameter, for mapping adomain of symbol sequence words to a co-domain of the integer-valuedsyntax elements. A reconstructor 605 is configured to reconstruct themedia data 606 based on the integer-valued syntax elements. Thedesymbolizer 602 is configured to perform the desymbolization such thatthe control parameter varies in accordance with the data stream at afirst rate in case of the high-efficiency mode being activated and thecontrol parameter is constant irrespective of the data stream or changesdepending on the data stream, but at a second rate lower than the firstrate in case of the low-complexity mode being activated, as it isillustrated by arrow 607. For example, the control parameter may vary inaccordance with previously desymbolized symbols.

Some of the above embodiments made use of the aspect of FIG. 19 . Thesyntax elements coeff_abs_minus3 and MVD within sequence 327 were, forexample, binarized in desymbolizer 314 depending on the mode selected asindicated by 407, and the reconstructor 605 used these syntax elementsfor reconstruction. Obviously, both aspects of FIGS. 18 and 19 arereadily combinable, but the aspect of FIG. 19 may also be combined withother coding environments.

See, for example, the motion vector difference coding denoted above. Thedesymbolizer 602 may be configured such that the mapping function uses atruncated unary code to perform the mapping within a first interval ofthe domain of integer-valued syntax elements below a cutoff value and acombination of a prefix in form of the truncated unary code for thecutoff value and a suffix in form of a VLC codeword within a secondinterval of the domain of integer-valued syntax elements inclusive andabove the cutoff value, wherein the decoder may comprise an entropydecoder 608 configured to derive a number of first bins of the truncatedunary code from the data stream 601 using entropy decoding with varyingprobability estimation and a number of second bins of the VLC codewordusing a constant equi-probability bypass mode. In HE mode, the entropycoding may be more complex than in LC coding as illustrated by arrow609. That is, context-adaptivity and/or probability adaptation may beapplied in HE mode and suppressed in LC mode, or the complexity may bescaled in other terms, as set out above with respect to the variousembodiments.

An encoder fitting to decoder of FIG. 18 , for encoding media data intoa data stream is shown in FIG. 20 . It may comprise an inserter 500configured to signal within the data stream 501 an activation of alow-complexity mode or a high efficiency mode, a constructor 504configured to precode the media data 505 into a sequence 506 of syntaxelements, a symbolizer 507 configured to symbolize the sequence 506 ofsyntax elements into a sequence 508 of symbols, a plurality of entropyencoders 310 each of which is configured to convert partial sequences ofsymbols into codewords of the data stream, and a selector 502 configuredto forward each symbol of the sequence 508 of symbols to a selected oneof the plurality of entropy encoders 310, wherein the selector 502 isconfigured to perform the selection depending on the activated one ofthe low complexity mode and the high-efficiency mode as illustrated byarrow 511. An interleaver 510 may be optionally provided forinterleaving the codewords of the encoders 310.

An encoder fitting to decoder of FIG. 19 , for encoding media data intoa data stream is shown in FIG. 21 as comprising an inserter 700configured to signal within the data stream 701 an activation of alow-complexity mode or a high efficiency mode, a constructor 704configured to precode the media data 705 into a sequence 706 of syntaxelements comprising an integer-valued syntax element, and a symbolizer707 configured to symbolize the integer-valued syntax element using amapping function controllable by a control parameter, for mapping adomain of integer-valued syntax elements to a co-domain of the symbolsequence words, wherein the symbolizer 707 is configured to perform thesymbolization such that the control parameter varies in accordance withthe data stream at a first rate in case of the high-efficiency modebeing activated and the control parameter is constant irrespective ofthe data stream or changes depending on the data stream, but at a secondrate lower than the first rate in case of the low-complexity mode beingactivated as illustrated by arrow 708. The symbolization result is codedinto the datastream 701.

Again, it should be mentioned that the embodiment of FIG. 18 is easilytransferable onto the above-mentioned context-adaptive binary arithmeticen/decoding embodiment: selector 509 and entropy encoders 310 wouldcondense into an entropy encoder 710 which would output the datastream401 directly and select the context for a bin currently to be derivedfrom the datastream. This is especially true for context adaptivityand/or probability adaptivity. Both functionalities/adaptivities may beswitched off, or designed more relaxed, during low complexity mode.

It is noted that many of the above aspects of the above embodiments arereadily transferable onto audio coding or coding of other data. Inparticular, the specific kind of reconstructor is not critical.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus. Some or all of the method steps may be executed by (or using)a hardware apparatus, like for example, a microprocessor, a programmablecomputer or an electronic circuit. In some embodiments, some one or moreof the most important method steps may be executed by such an apparatus.

The inventive encoded signal can be stored on a digital storage mediumor can be transmitted on a transmission medium such as a wirelesstransmission medium or a wired transmission medium such as the Internet.

Depending on certain implementation requirements, embodiments of theinvention can be implemented in hardware or in software. Theimplementation can be performed using a digital storage medium, forexample a floppy disk, a DVD, a Blue-Ray, a CD, a ROM, a PROM, an EPROM,an EEPROM or a FLASH memory, having electronically readable controlsignals stored thereon, which cooperate (or are capable of cooperating)with a programmable computer system such that the respective method isperformed. Therefore, the digital storage medium may be computerreadable.

Some embodiments according to the invention comprise a data carrierhaving electronically readable control signals, which are capable ofcooperating with a programmable computer system, such that one of themethods described herein is performed.

Generally, embodiments of the present invention can be implemented as acomputer program product with a program code, the program code beingoperative for performing one of the methods when the computer programproduct runs on a computer. The program code may for example be storedon a machine readable carrier.

Other embodiments comprise the computer program for performing one ofthe methods described herein, stored on a machine readable carrier.

In other words, an embodiment of the inventive method is, therefore, acomputer program having a program code for performing one of the methodsdescribed herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a datacarrier (or a digital storage medium, or a computer-readable medium)comprising, recorded thereon, the computer program for performing one ofthe methods described herein. The data carrier, the digital storagemedium or the recorded medium are typically tangible and/ornon-transitionary.

A further embodiment of the inventive method is, therefore, a datastream or a sequence of signals representing the computer program forperforming one of the methods described herein. The data stream or thesequence of signals may for example be configured to be transferred viaa data communication connection, for example via the Internet.

A further embodiment comprises a processing means, for example acomputer, or a programmable logic device, configured to or adapted toperform one of the methods described herein.

A further embodiment comprises a computer having installed thereon thecomputer program for performing one of the methods described herein.

A further embodiment according to the invention comprises an apparatusor a system configured to transfer (for example, electronically oroptically) a computer program for performing one of the methodsdescribed herein to a receiver. The receiver may, for example, be acomputer, a mobile device, a memory device or the like. The apparatus orsystem may, for example, comprise a file server for transferring thecomputer program to the receiver.

In some embodiments, a programmable logic device (for example a fieldprogrammable gate array) may be used to perform some or all of thefunctionalities of the methods described herein. In some embodiments, afield programmable gate array may cooperate with a microprocessor inorder to perform one of the methods described herein. Generally, themethods may be performed by any hardware apparatus.

The above described embodiments are merely illustrative for theprinciples of the present invention. It is understood that modificationsand variations of the arrangements and the details described herein willbe apparent to others skilled in the art. It is the intent, therefore,to be limited only by the scope of the impending patent claims and notby the specific details presented by way of description and explanationof the embodiments herein.

Thus, inter alias, the above description revealed a decoder for decodinga data stream 401 into which media data is coded, comprising a modeswitch 400 configured to activate a low-complexity mode or a highefficiency mode depending on the data stream; a plurality of entropyencoders 322 each of which is configured to convert codewords in thedata stream to partial sequences 321 of symbols; a selector 402configured to retrieve each symbol of a sequence 326 of symbols from aselected one of the plurality of entropy decoders 322; a desymbolizer314 configured to desymbolize the sequence of symbols 326 in order toobtain a sequence of syntax elements 327; a reconstructor 404 configuredto reconstruct the media data based on the sequence of syntax elements;wherein the selector 402 is configured to perform the selectiondepending on the activated one of the low complexity mode and thehigh-efficiency mode. The selector 402 may be configured to perform, fora predetermined symbol, the selection among the plurality of entropydecoders 322 depending on previously retrieved symbols of the sequence326 of symbols in case of the high-efficiency mode being activated andindependent from any previously retrieved symbols of the sequence ofsymbols in case of the low-complexity mode being activated. The datastream may be structured into consecutive portions and each symbol ofthe sequence of symbols may be associated with a respective one of aplurality of symbol types, wherein the selector 402 may be configured tovary, for symbols of a predetermined symbol type within a currentportion, the selection depending on previously retrieved symbols of thesequence of symbols of the predetermined symbol type within the currentportion in case of the high-efficiency mode being activated, and leavethe selection constant within the current portion in case of thelow-complexity mode being activated. It is also feasible that eachsymbol of the sequence 326 of symbols is associated with a respectiveone of a plurality of symbol types, wherein the selector 402 isconfigured to, for a predetermined symbol of a predetermined symboltype, select one of a plurality of contexts depending on previouslyretrieved symbols of the sequence of symbols and perform the selectionamong the entropy decoders 322 depending on a probability modelassociated with the selected context along with updating the probabilitymodel associated with the selected context depending on thepredetermined symbol in case of the high-efficiency mode beingactivated, and perform selecting the one of the plurality of contextsdepending on the previously retrieved symbols of the sequence of symbolsand perform the selection among the entropy decoders 322 depending onthe probability model associated with the selected context along withleaving the probability model associated with the selected contextconstant in case of the low-complexity mode being activated.Alternatively or additionally, each symbol of the sequence 326 ofsymbols may be associated with a respective one of a plurality of symboltypes, wherein the selector 402 may be configured to, for each symbol ofa predetermined symbol type, perform the selection among the entropydecoders 322 depending on a probability model associated with thepredetermined symbol type along with updating the probability modeldepending on the symbols of the predetermined symbol type at a firstupdate rate in case of the high-efficiency mode being activated, and foreach symbol of the predetermined symbol type, perform the selectionamong the entropy decoders 322 depending on the probability model, withintermittently updating the probability model depending on the symbolsof the predetermined symbol type at a second update rate lower than thefirst update rate in case of the low-complexity mode being activated. Itis also feasible that the selector 402 is configured to perform, forsymbols of a predetermined symbol type, a probability mode adaptationusing a probability state index defined at a first probability stateaccuracy and perform the selection among the entropy decoders 322depending on the probability state index defined at the firstprobability state accuracy in case of the high-efficiency mode beingactivated, and perform no probability mode adaptation or a probabilitymode adaptation using a probability state index defined at a secondprobability state accuracy lower than the first probability stateaccuracy and perform the selection among the entropy decoders dependingon the probability state index defined at the second probability stateaccuracy in case of the low-complexity mode being activated. It is alsofeasible that the reconstructor 404 is configured to operate independentfrom the high-efficiency mode or the low-complexity mode beingactivated. The reconstructor 404 may be configured to reconstruct atransform block 200 of transform coefficient levels 202 based on aportion of the sequence of syntax elements independent from thehigh-efficiency mode or the low-complexity mode being activated, theportion of the sequence of syntax elements comprising, in anun-interleaved manner, significance map syntax elements defining asignificance map indicating positions of non-zero transform coefficientlevels within the transform block 200; and level syntax elementsdefining the non-zero transform coefficient levels. The reconstructor404 may be configured to reconstruct a transform block 200 of transformcoefficient levels 202 based on a portion of the sequence of syntaxelements independent from the high-efficiency mode or the low-complexitymode being activated, the portion of the sequence of syntax elementscomprising, in an un-interleaved manner, end position syntax elements(last_signficant_pos_x, last_signficant_pos_y) indicating a position ofa last non-zero transform coefficient level within the transform block;first syntax elements (coeff_signficant_flag) together defining asignificance map and indicating, for each position along aone-dimensional path 274 leading from a DC position to the position ofthe last non-zero transform coefficient level within the transform block200, as to whether the transform coefficient level at the respectiveposition is non-zero or not; second syntax elements (coeff_abs_greater1)indicating, for each position of the one-dimensional path (274) where,according to the first binary syntax elements, a non-zero transformcoefficient level is positioned, as to whether the transform coefficientlevel at the respective position is greater than one; third syntaxelements (coeff_abs_greater2. coeff_abs_minus3) revealing, for eachposition of the one-dimensional path where, according to the firstbinary syntax elements, a transform coefficient level greater than oneis positioned, an amount by which the respective transform coefficientlevel at the respective position exceeds one, wherein an order among theend positions syntax elements, and the first, second and third syntaxelements is the same for the high-efficiency mode and the low-complexitymode, and wherein the selector 402 is configured to perform theselection among the entropy decoders 322 for symbols from which thedesymbolizer obtains the end positions syntax elements, first syntaxelements, second syntax elements and/or the third syntax elements,differently depending on the complexity mode or the high-efficiency modebeing activated. In this regard, the selector 402 may be configured to,for symbols of a predetermined symbol type among a subsequence ofsymbols from which the desymbolizer obtains the first syntax elementsand second syntax elements, select for each symbol of the predeterminedsymbol type one of a plurality of contexts depending on previouslyretrieved symbols of the predetermined symbol type among the subsequenceof symbols and perform the selection depending on a probability modelassociated with the selected context in case of the high-efficiency modebeing activated, and perform the selection in a piecewise constantmanner such that the selection is constant over consecutive continuoussubparts of the subsequence in case of the low-complexity mode beingactivated. The selector 402 is configured to, for the symbols of thepredetermined symbol type among the subsequence of symbols from whichthe desymbolizer obtains the first syntax elements and second syntaxelements, select for each symbol of the predetermined symbol type theone of a plurality of contexts depending on a number of previouslyretrieved symbols of the predetermined symbol type within thesubsequence of symbols, which have a predetermined symbol value andbelong to the same subpart, or a number of previously retrieved symbolsof the predetermined symbol type within the subsequence of symbols,which belong to the same subpart. The third syntax elements revealing,for each position of the one-dimensional path where, according to thefirst binary syntax elements, a transform coefficient level greater thanone is positioned, an amount by which the respective transformcoefficient level at the respective position exceeds one, may compriseinteger-valued syntax elements (coeff_abs_minus3), and the desymbolizer314 may be configured to use a mapping function controllable by acontrol parameter to map a domain of symbol sequence words to aco-domain of the integer-valued syntax elements, and to set the controlparameter per integer-valued syntax element depending on integer-valuedsyntax elements of previous third syntax elements if the high-efficiencymode is activated, and perform the setting in a piecewise constantmanner such that the setting is constant over consecutive continuoussubparts of the subsequence in case of the low-complexity mode beingactivated, wherein the selector 402 is configured to select apredetermined one of the entropy decoders 322 for the symbols of symbolsequence words mapped onto the integer-valued syntax elements, which isassociated with an equal probability distribution, in both thehigh-efficiency mode and the low-complexity mode. Each symbol of thesequence 326 of symbols may be associated with a respective one of aplurality of symbol types, wherein the selector 402 may be configuredto, for symbols of each of a number of predetermined symbol types,perform the selection among the entropy decoders 322 depending on arespective probability model associated the respective predeterminedsymbol such that the number of predetermined symbol types is lower inthe low complexity mode than compared to the high-efficiency mode. Eachsymbol of the sequence 326 of symbols may be associated with arespective one of a plurality of symbol types, wherein the selector 402may be configured to, for symbols of a predetermined symbol type,perform the selection among the entropy decoders 322 depending on aprobability model associated with the predetermined symbol type alongwith or without updating the associated probability model, such that alength of a learning phase of the sequence of symbols over which theselection for the symbols of the predetermined symbol type is performedalong with the update, is shorter in the low complexity mode thancompared to the high-efficiency mode. Each symbol of the sequence 326 ofsymbols may be associated with a respective one of a plurality of symboltypes, wherein the selector 402 may be configured to, for symbols of apredetermined symbol type, perform the selection among the entropydecoders 322 depending on a probability model associated with thepredetermined symbol type along with or without updating the associatedprobability model such that a frequency at which the selection for thesymbols of the predetermined symbol type is performed along with theupdate, is lower in the low complexity mode than compared to thehigh-efficiency mode. Each symbol of the sequence 326 of symbols may beassociated with a respective one of a plurality of symbol types, whereinthe selector 402 is configured to, for symbols of a predetermined symboltype, select one of a number of contexts depending on a number ofpreviously retrieved symbols of the sequence of symbols and perform theselection among the entropy decoders 322 depending on a probabilitymodel associated with the selected context, such that the number ofcontexts, and/or the number of previously retrieved symbols, is lower inthe low complexity mode than compared to the high-efficiency mode. Eachsymbol of the sequence 326 of symbols may be associated with arespective one of a plurality of symbol types, wherein the selector 402may be configured to, for symbols of a predetermined symbol type,perform the selection among the entropy decoders 322 depending on aprobability model associated with the predetermined symbol type, alongwith updating the associated probability model every x-th symbol of thepredetermined type based on y most recent symbols of the predeterminedsymbol type such that the ratio x/y is higher in the low complexity modethan compared to the high-efficiency mode. Each symbol of the sequence326 of symbols may be associated with a respective one of a plurality ofsymbol types, wherein the selector 402 may be configured to initializeprobability models associated with the symbol types based on acomputation using syntax elements in the data stream which computationand syntax elements are the same in the low complexity mode and thehigh-efficiency mode, respectively, with, however, a resolution of aresult of the computation being lower in the low complexity mode and thehigh-efficiency mode.

While this invention has been described in terms of several embodiments,there are alterations, permutations, and equivalents which will beapparent to others skilled in the art and which fall within the scope ofthis invention. It should also be noted that there are many alternativeways of implementing the methods and compositions of the presentinvention. It is therefore intended that the following appended claimsbe interpreted as including all such alterations, permutations, andequivalents as fall within the true spirit and scope of the presentinvention.

1-20. (canceled)
 21. A decoder for decoding a data stream includingencoded data of a video, the decoder comprising: an entropy decodingengine configured to decode data from the data stream based on aselected one of a plurality of entropy decoding schemes to obtain asequence of symbols, wherein the selected one of the plurality ofentropy decoding schemes includes a context adaptive binary arithmeticcoding scheme, and wherein the selected one of the plurality of entropydecoding schemes is selected based on activation of one of a lowcomplexity mode and a high-efficiency mode, the plurality of entropydecoding schemes differ from each other in terms of a differentprobability estimate used in corresponding arithmetic decoding, and withrespect to at least one symbol of the sequence of symbols, the entropydecoding engine is configured to: select a context corresponding to theat least one symbol, the context having associated therewith aprobability model, decode the at least one symbol using the contextbased on the selected one of the plurality of entropy decoding schemes,and update the probability model; and a desymbolizer configured todesymbolize the sequence of symbols to obtain a sequence of syntaxelements.
 22. The decoder according to claim 21, wherein the at leastone symbol is associated with one of a plurality of symbol types, andthe entropy decoding engine is configured to select the context based onpreviously decoded symbols of the symbol type associated with the atleast one symbol.
 23. The decoder according to claim 21, wherein theprobability model is associated with a probability state index having afirst probability state accuracy for the high-efficiency mode, and asecond probability state accuracy, lower than the first probabilitystate accuracy, for the low complexity mode.
 24. The decoder accordingto claim 21, wherein the data stream comprises information associatedwith color samples of the video.
 25. The decoder according to claim 21,wherein the data stream comprises information associated with a depthmap of the video.
 26. The decoder according to claim 21, wherein theselected one of the plurality of entropy decoding schemes is selectedbased on previously retrieved symbols of the sequence of symbols in caseof the high-efficiency mode being activated and independent from anypreviously retrieved symbols of the sequence of symbols in case of thelow-complexity mode being activated.
 27. The decoder according to claim21, wherein the data stream is structured into consecutive portions andeach symbol of the sequence of symbols is associated with a respectiveone of a plurality of symbol types, wherein, for symbols of apredetermined symbol type within a current portion, selection of theselected one of the plurality of entropy decoding schemes variesdepending on previously retrieved symbols of the sequence of symbols ofthe predetermined symbol type within the current portion in case of thehigh-efficiency mode being activated, and is left constant within thecurrent portion in case of the low-complexity mode being activated. 28.The decoder according to claim 21, wherein each symbol of the sequenceof symbols is associated with a respective one of a plurality of symboltypes, wherein the entropy decoding engine is configured to, for apredetermined symbol of a predetermined symbol type, select the contextdepending on previously retrieved symbols of the sequence of symbols,and wherein: selection of the selected one of the plurality of entropydecoding schemes depends on the probability model associated with thecontext along with updating the probability model associated with thecontext depending on the predetermined symbol in case of thehigh-efficiency mode being activated, and selection of the selected oneof the plurality of entropy decoding schemes depends on the probabilitymodel associated with the context along with leaving the probabilitymodel associated with the context constant in case of the low-complexitymode being activated.
 29. The decoder according to claim 21, whereineach symbol of the sequence of symbols is associated with a respectiveone of a plurality of symbol types, wherein selection of the selectedone of the plurality of entropy decoding schemes depends, for symbols ofeach of a number of predetermined symbol types, on a respectiveprobability model associated the respective predetermined symbol suchthat the number of predetermined symbol types is lower in the lowcomplexity mode than compared to the high-efficiency mode.
 30. Thedecoder according to claim 21, wherein each symbol of the sequence ofsymbols is associated with a respective one of a plurality of symboltypes, wherein selection of the selected one of the plurality of entropydecoding schemes depends, for symbols of a predetermined symbol type, ona probability model associated with the predetermined symbol type alongwith or without updating the associated probability model, such that alength of a learning phase of the sequence of symbols over which theselection for the symbols of the predetermined symbol type is performedalong with the update, is shorter in the low complexity mode thancompared to the high-efficiency mode.
 31. The decoder according to claim21, wherein each symbol of the sequence of symbols is associated with arespective one of a plurality of symbol types, wherein selection of theselected one of the plurality of entropy decoding schemes depends, forsymbols of a predetermined symbol type, on a probability modelassociated with the predetermined symbol type along with or withoutupdating the associated probability model such that a frequency at whichthe selection for the symbols of the predetermined symbol type isperformed along with the update, is lower in the low complexity modethan compared to the high-efficiency mode.
 32. An encoder for encodingdata of a video into a data stream, the encoder comprising: aconstructor configured to precode the data of the video into a sequenceof syntax elements; a symbolizer configured to symbolize the sequence ofsyntax elements into a sequence of symbols; and an entropy encodingengine configured to encode each symbol of the sequence of symbols intothe data stream using a selected one of a plurality of entropy encodingschemes, wherein the selected one of the plurality of entropy encodingschemes includes a context adaptive binary arithmetic coding scheme, andwherein the selected one of the plurality of entropy decoding schemes isselected based on activation a low complexity mode or a high-efficiencymode, the plurality of entropy decoding schemes differ from each otherin terms of a different probability estimate used in correspondingarithmetic encoding, and with respect to at least one symbol of thesequence of symbols, the entropy encoding engine is configured to:select a context corresponding to the at least one symbol, the contexthaving associated therewith a probability model, encode the at least onesymbol using the context based on the selected one of the plurality ofentropy encoding schemes, and update the probability model.
 33. Theencoder according to claim 32, wherein the at least one symbol isassociated with one of a plurality of symbol types, and the entropyencoding engine is configured to select the context based on previouslyencoded symbols of the symbol type associated with the at least onesymbol.
 34. The encoder according to claim 32, wherein the data streamcomprises information associated with color samples of the video. 35.The encoder according to claim 32, wherein the data stream comprisesinformation associated with a depth map of the video.